When loading the map file for a DOL/ELF file, extract only the filename from the DOL/ELF's path

Previously Dolphin would, for an ELF located at C:\subdirectory\test.elf, try to load its map file from C:\Users\username\Dolphin Emulator\Maps\C:\subdirectory\test.map.

This patch strips off the directories and uses only the name to create the path to the .map file, which means that, for the example above, the map file will be loaded from C:\Users\username\Dolphin Emulator\Maps\test.map instead.

The string manipulation used here is a little ugly - do I need to clean it up a bit?
std::string's find returns -1 for "not found"; having a signed variable helps with calculating the index if not found is returned (since comparisons would still work properly)