Create markable/content-based hex editor scroll bar

[nyxxxie]

Create a scroll bar that allows for marks to be added to and removed from it. For example, this scrollbar would allow us to mark the position on the scrollbar where a search has found results. It would also be neat if we made a scroll bar that showed the colors of struct-defined areas in a file.

• existing impl: https://freddie.witherden.org/tools/libqmarkedscrollbar/doxygen/class_marked_scroll_bar.html