plugin-mainmenu: Use groupbox and simplify config GUI

[isf63]

This is somewhat unimportant due to Fancy Menu being the new default, but it reduces GUI code and should speed up builds.

Arguably it makes the GUI more clear, unless nested groupboxes are a point of confusion.

[isf63]

Editing the .cpp file revealed that static_cast and QOverload<> for signals are used throughout the whole source. They can be removed due to Qt6 improvements with the goal of lighter compiling.

[tsujan]

Editing the .cpp file revealed that static_cast and QOverload<> for signals are used throughout the whole source.

They're remnants of old times. They're safe, but I remove them in sources that I work on (in my PRs).

I'll review this PR later, being madly busy for now.

[isf63]

Compilation would be sped up by removing all unnecessary QOverloads/signal casts, because they're complex templates, no? My motivation is to improve compilation times despite lxqt-panel's innate complexity.

[tsujan]

Compilation would be sped up

Not in reality.