Scope markers
Added a marker feature to the scope displays. Implements #497. See https://github.com/f4exb/sdrangel/blob/master/plugins/channelrx/chanalyzer/readme.md#g-markers for details. Applies also to other Scope instances for example the ATV demod scope display.
Spectrum markers
Added a marker feature for spectrum displays. Implements #557. See https://github.com/f4exb/sdrangel/blob/master/sdrgui/readme.md#spectrum-markers for details. Applies to all spectrum instances.
Other changes and fixes
- ScopeVis: fixed moving of iterator past the end. Fixes issue #556