Add a new DynamicMeter class for runtime Meter extension

This commit is based on exploratory work by Sohaib Mohamed.
The end goal is two-fold - to support addition of Meters we
build via configuration files for both the PCP platform and
for scripts ( https://github.com/htop-dev/htop/issues/526 )

Here, we focus on generic code and the PCP support.  A new
class DynamicMeter is introduced - it uses the special case
'param' field handling that previously was used only by the
CPUMeter, such that every runtime-configured Meter is given
a unique identifier.  Unlike with the CPUMeter this is used
internally only.  When reading/writing to htoprc instead of
CPU(N) - where N is an integer param (CPU number) - we use
the string name for each meter.  For example, if we have a
configuration for a DynamicMeter for some Redis metrics, we
might read and write "Dynamic(redis)".  This identifier is
subsequently matched (back) up to the configuration file so
we're able to re-create arbitrary user configurations.

The PCP platform configuration file format is fairly simple.
We expand configs from several directories, including the
users homedir alongside htoprc (below htop/meters/) and also
/etc/pcp/htop/meters.  The format will be described via a
new pcp-htop(5) man page, but its basically ini-style and
each Meter has one or more metric expressions associated, as
well as specifications for labels, color and so on via a dot
separated notation for individual metrics within the Meter.

A few initial sample configuration files are provided below
./pcp/meters that give the general idea.  The PCP "derived"
metric specification - see pmRegisterDerived(3) - is used
as the syntax for specifying metrics in PCP DynamicMeters.

DynamicMeter.c

@@ -0,0 +1,98 @@
+/*
+htop - DynamicMeter.c
+(C) 2021 htop dev team
+(C) 2021 Red Hat, Inc.  All Rights Reserved.
+Released under the GNU GPLv2, see the COPYING file
+in the source distribution for its full text.
+*/
+#include "config.h" // IWYU pragma: keep
+
+#include "DynamicMeter.h"
+
+#include "CRT.h"
+#include "Object.h"
+#include "Platform.h"
+#include "ProcessList.h"
+#include "RichString.h"
+#include "XUtils.h"
+
+
+static const int DynamicMeter_attributes[] = {
+   DYNAMIC_GRAY,
+   DYNAMIC_DARKGRAY,
+   DYNAMIC_RED,
+   DYNAMIC_GREEN,
+   DYNAMIC_BLUE,
+   DYNAMIC_CYAN,
+   DYNAMIC_MAGENTA,
+   DYNAMIC_YELLOW,
+   DYNAMIC_WHITE
+};
+
+Hashtable* DynamicMeters_new(void) {
+   return Platform_dynamicMeters();
+}
+
+typedef struct {
+   unsigned int key;
+   const char* name;
+} DynamicIterator;
+
+static void DynamicMeter_compare(ht_key_t key, void* value, void* data) {
+   const DynamicMeter* meter = (const DynamicMeter*)value;
+   DynamicIterator* iter = (DynamicIterator*)data;
+   if (String_eq(iter->name, meter->name))
+      iter->key = key;
+}
+
+unsigned int DynamicMeter_search(const ProcessList* pl, const char* name) {
+   DynamicIterator iter = { .key = 0, .name = name };
+   if (pl->dynamicMeters)
+      Hashtable_foreach(pl->dynamicMeters, DynamicMeter_compare, &iter);
+   return iter.key;
+}
+
+const char* DynamicMeter_lookup(const ProcessList* pl, unsigned int key) {
+   const DynamicMeter* meter = Hashtable_get(pl->dynamicMeters, key);
+   return meter ? meter->name : NULL;
+}
+
+static void DynamicMeter_init(Meter* meter) {
+   Platform_dynamicMeterInit(meter);
+}
+
+static void DynamicMeter_updateValues(Meter* meter) {
+   Platform_dynamicMeterUpdateValues(meter);
+}
+
+static void DynamicMeter_display(const Object* cast, RichString* out) {
+   const Meter* meter = (const Meter*)cast;
+   Platform_dynamicMeterDisplay(meter, out);
+}
+
+static void DynamicMeter_getUiName(const Meter* this, char* name, size_t length) {
+   const ProcessList* pl = this->pl;
+   const DynamicMeter* meter = Hashtable_get(pl->dynamicMeters, this->param);
+   if (meter) {
+      const char* uiName = meter->caption ? meter->caption : meter->name;
+      xSnprintf(name, length, "%s", uiName);
+   }
+}
+
+const MeterClass DynamicMeter_class = {
+   .super = {
+      .extends = Class(Meter),
+      .delete = Meter_delete,
+      .display = DynamicMeter_display
+   },
+   .init = DynamicMeter_init,
+   .updateValues = DynamicMeter_updateValues,
+   .getUiName = DynamicMeter_getUiName,
+   .defaultMode = TEXT_METERMODE,
+   .maxItems = 0,
+   .total = 100.0,
+   .attributes = DynamicMeter_attributes,
+   .name = "Dynamic",
+   .uiName = "Dynamic",
+   .caption = "",
+};