The original reason I wanted to do this was so that we can replace
the Android-specific code with this in the future, but of course,
just deduplicating between DolphinWX and DolphinQt2 is nice too.

Fixes:

- DolphinQt2 showing the wrong size for split WBFS disc images.

- DolphinQt2 being case sensitive when checking if a file is a DOL/ELF.

- DolphinQt2 not detecting when a Wii banner has become available
after the game list cache was created.

Removes:

- DolphinWX's ability to load PNGs as custom banners. But it was
already rather broken (see https://bugs.dolphin-emu.org/issues/10365
and https://bugs.dolphin-emu.org/issues/10366). The reason I removed
this was because PNG decoding relied on wx code and we don't have any
good non-wx/Qt code for loading PNG files right now (let's not use
SOIL), but we should be able to use libpng directly to implement PNG
loading in the future.

- DolphinQt2's ability to ignore a cached game if the last modified
time differs. We currently don't have a non-wx/Qt way to get the time.

Setting a single icon at a single resolution doesn't scale well,
Windows requires a 16x16 icon for the window and a 32x32/48x48 for
the taskbar. Providing all icons produces less pixellated results at
HiDPI.

Changes:
  - MemoryWindow was cleaned up and gives more feedback on searches.

Some bugs were fixed as well:
  - A complex bug that allowed tearing off tabs and opening multiple
    copies of a debug panel which lead to segfaults
  - Another segfault related to right-click menus on code/memory views
    when those tools were floating in their own window.

Portable flexible HiDPI image loading and other support for Windows/GTK/OSX.

Using the XPM format for images has become a maintenance problem because
people don't know how to create them. This commit removes all XPM images
and all C files that contain PNG images. DolphinWX now uses the PNGs
in the Resources folder instead, just like DolphinQt and DolphinQt2 do.