Memory: Handle RasterizerCachedMemory and RasterizerCachedSpecial page types in the memory block manipulation functions.

Also rewrite Qt wrappers to use those.

This allows frontends to keep a single loader and use it multiple times
e.g. for code loading and SMDH parsing.

Fixes #1115.

Also improves the performances of DiskArchive’s directory
implementation a lot, simply by not going through the entire tree
instead of just listing the first level files.

Thanks to JayRoxFox for rebasing this on current master!