Add monitor volume adjustment

[tmiw]

Resolves #732 by adding new right-click menu items for adjusting monitoring volume under PTT and Voice Keyer. Example:

Clicking outside of the box that appears will close it. Additionally, updates here are live (i.e. if you're transmitting, you can hear the volume changing as you adjust the slider).

[barjac]

Very nice!
However the slider widget melts into the display elements behind it especially when the main window is using multiple widget columns either side of the scope.

Is it possible to put a border line around it?
Maybe even dim the whole display behind it while it has focus?
Or maybe give it a different background colour or invert it's foreground/background?

[Tyrbiter]

The only thing I will mention as a minor thing, the new monitor settings and code have the word Monitor typo'd as Montior, it's not urgent but probably best to fix it sooner rather than later.

Will report on the actual function of the change later.

[tmiw]

Is it possible to put a border line around it? Maybe even dim the whole display behind it while it has focus? Or maybe give it a different background colour or invert it's foreground/background?

There's a border around it now as of 09b7275.

The only thing I will mention as a minor thing, the new monitor settings and code have the word Monitor typo'd as Montior, it's not urgent but probably best to fix it sooner rather than later.

Fixed.

[barjac]

Yes that looks and works fine for me now.
Nice job Mooneer! :)