citra-qt: Rewrite disassembly view.

Changed the underlying Qt code to not reinvent the wheel in the most stupid way possible and to actually use Qt's model/view framework instead. That should be good for performance, even though it still struggles when browsing large chunks of disassembly >= 1 MB in size.

Other than that, slightly changed the colors of the breakpoint highlighting (RGB 0xff 0xc0 0xc0 now) and re-added the "next instruction" highlighting (RGB 0xc0 0xc0 0xff).