GH-3068 Fix icons overwriting each other if not needed

[Zetabite]

When importing icons, mmc will now create an icon, with the filename and the md5 hash of the file appeneded to it. icon2.png becomes icon2_4190adsawewq.png or similar. In case a file with that name already exists, both files are checked on their md5sum, if they equal, the new file isnt imported. If the files have different checksums, the existing file becomes reevaluated (should only happen if someone purpoesfully adds a wrong hash to a file ) and written to the actual name it should have. ideally, that should never happen.