Migrate PNGs from DolphinWX to the Data/Sys/Resources directory.

 * Aren't used by anything (yet) as DolphinWX compiles XPM versions into the build
 * Moved the .desktop file as well, adjusted INSTALL rule accordingly
Merge pull request #998 from waddlesplash/wx-fixup

Migrate PNGs from DolphinWX to the Data/Resources directory.