Don't allow unselecting paint style buttons.

Set group for style buttons to make them behave like radio buttons.

Considered whether it's possible to get rid of set_style_buttons now that the single selection is handled by GTK. While it could be possible to replace most set_style_buttons(x) calls with x->set_active(true), there is no clean way to unselect all used by set_mode_multiple (at least I couldn't spot anything useful in GTK docs). So for now chose keeping MR as simple as possible without additional refactoring.

Closes #5758 (closed)