Pcbnew: Allow placing bitmaps initial hack

This branch is just for discussion at this point. I pretty much just copy and pasted the bitmap code from the schematic editor to get bitmaps onto the PCB canvas. I personally would use it for crappy datasheets with difficult footprints to draw and perhaps some mechanical item checking where a 3D model is unavailable.

Some bugs this would fix: Fixes: #2054 (closed)

Discussion of how some users would like this feature to work: #4689 (closed)

There are a lot of very specific use cases 4689 laid out. I think allowing the users to place images on the User.X layers should be mostly sufficient. Users can already flip the board, control transparency, and build custom layer presets that should accomplish most of what this is asking for.