Basic .sym support

This adds basic .sym support, a format originally used by the no$ emulators, which is also generated by some assemblers. It's a very simple format that just maps addresses to symbols.

The format is basically like this:
%08X name

8 digits the address, then the symbol name after a space. It supports specifying data regions too, but those aren't supported yet, I will need to do bigger changes to the disassembly view to support and display arbitrary sizes.

Additionally, it now also supports loading symbols for virtual discs. It loads them from dir/.ppsspp-symbols.map or  dir/.ppsspp-symbols.sym.