Add new display option to also show CPU frequency in CPU meters.

The option is only implemented on Linux. On other platforms, and on Linuxes
that do not expose the relevant sysfs file, the frequency will be 0.

The "CPU average" meter does not show a frequency, only
the individual per-CPU meters.

Settings.h

@@ -36,6 +36,7 @@ typedef struct Settings_ {
 
    bool countCPUsFromZero;
    bool detailedCPUTime;
+   bool showCPUFrequency;
    bool treeView;
    bool showProgramPath;
    bool hideThreads;