Color config dialog change + minor bugfixes

[dendvz]

1. Enable user to choose screen & print colors separately (option already supported by DsoSettingsView.color).

2. Enabled "Export Images with Screen Colors" checkbox in "Files" dialog.

3. Couple of minor enhancements: fixed width of icons panel in Settings, suppress printing spectrum magnitude if spectrum is not printed.

4. Bugfixes:
   Eliminated compile time warnings:

...helper.h:111:2: warning: extra ‘;’ [-Wpedantic]
 };
  ^
...control.cpp: In member function ‘unsigned int Hantek::Control::updateSamplerate(unsigned int, bool)’:
...control.cpp:724:26: warning: overflow in implicit constant conversion [-Woverflow]
         downsamplerValue = 0xffff;
                          ^