Add a date and datetime meter (#159)

Add a date meter and sort header and source files in Makefile

Change the lists of header and source files sorted alphabetical and one
file per line. This way diffs become better readable and merges easier.

DateMeter.c

@@ -0,0 +1,47 @@
+/*
+htop - DateMeter.c
+(C) 2004-2020 Hisham H. Muhammad, Michael Schönitzer
+Released under the GNU GPL, see the COPYING file
+in the source distribution for its full text.
+*/
+
+#include "DateMeter.h"
+
+#include "CRT.h"
+
+#include <time.h>
+
+
+int DateMeter_attributes[] = {
+   DATE
+};
+
+static void DateMeter_updateValues(Meter* this, char* buffer, int size) {
+   time_t t = time(NULL);
+   struct tm result;
+   struct tm *lt = localtime_r(&t, &result);
+   this->values[0] = lt->tm_yday;
+   int year = lt->tm_year + 1900;
+   if (((year % 4 == 0) && (year % 100!= 0)) || (year%400 == 0)) {
+      this->total = 366;
+   }
+   else {
+      this->total = 365;
+   }
+   strftime(buffer, size, "%F", lt);
+}
+
+MeterClass DateMeter_class = {
+   .super = {
+      .extends = Class(Meter),
+      .delete = Meter_delete
+   },
+   .updateValues = DateMeter_updateValues,
+   .defaultMode = TEXT_METERMODE,
+   .maxItems = 1,
+   .total = 365,
+   .attributes = DateMeter_attributes,
+   .name = "Date",
+   .uiName = "Date",
+   .caption = "Date: ",
+};