Fix building with MinGW

- Include string.h for strdup()
- Fix warning with comparison between unsigned and signed ints
Use %APPDATA% for settings/saves on Windows
Fix regression in MenuInventory.cpp

I forgot to account for item stacks that are dragged in from
MenuStash/MenuVendor.
Merge pull request #1122 from dorkster/fixes

Fix regression in MenuInventory.cpp
Merge pull request #1123 from dorkster/windows

Use %APPDATA% for settings/saves on Windows