Add appmenu scrollbar functionality

[4ctiv]

Added scrollbar functionatlity to graphical appmenu of instantOS to enhance touch screen experience by enabling to scroll without needing keyboard / mouse input.

Note: This will overwrite scrolling from left to right to be scrolling up and down instead.

[paperbenni]

That's a really nice idea, unfortunately rofi scrollbars seem to be broken at the moment, at least in combination with the rest of appmenu.rasi. On my machine dragging the scroll indicator causes it to become stuck on the mouse cursor and moving the cursor around in that state causes performance to progressively get worse until rofi does not respond anymore and either closes itself after waiting a while or needs to be killed via tty. Again, great idea but we'll have to wait until this is fixed in rofi