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.

GameTracker's usage of GameFileCache is thread-safe even without
using a mutex. All of its access to GameFileCache happens on the
thread m_load_thread, except for the call to GameFileCache::Load,
which finishes before m_load_thread starts executing.

This updates the maker data to (mostly) mirror that of the Wiki:
https://wiki.dolphin-emu.org/index.php?title=GameIDs

Only maker ids from that page are now included in Dolphin. This
means no homebrew/unofficial makers.

Also, separate multiple maker names with a slash

Some of my WiiWare games does not have a maker :
- Blue's Journey : EAFPJ8
- Magician Lord : EACPJ8
- The King of Fighters '94 : EAGPJ8
- The Last Ninja : C9XPGX
- World Games : C9ZPGX