add an option "flat_button" which acts in a similar way to the same option in xfce-panel's tasklist plugin

The single reason why I needed this was I was trying to theme the button that whiskermenu adds to the panel.

I was trying to add a transparent png with the pixmap engine, and thus needed to theme "state = NORMAL", however it was hard-coded as GTK_RELIEF_NONE. Rather than just changing the value manually in the code, I added a single checkbox next to the Display options for the icon.

Checking it sets the button's relief to GTK_RELIEF_NONE (by default, and that's the current setting)
Unchecking it sets the button's relief to GTK_RELIEF_NORMAL