[SUG][PLAYER] Allow customisation of "button selected" graphic (panel)

[WillLem]

This one was talked about on Discord and it was suggested that I create a topic for it, so here goes!

Just wondering if it's possible to add the "button selected" graphic (currently a white rectangle) to gfx/panel and panel-hr to make it a customisable image. This way, all graphical elements of the panel can be edited.

[namida]

As I already indicated on Discord, this feature is pretty much a "will happen, the topic just needs to be created so I don't forget".

[namida]

Implemented in commit fdee510, with a few fixes in 3f81bc9.