Find a better way to keep track of custom mimetypes
Because of https://bugs.kde.org/show_bug.cgi?id=492006, I realized that we don't really have a good way to keep track of custom mimetypes. Whenever someone creates a custom mimetype for clipboard items, they have to know that they may need to update paste.cpp or else you'll get junk like "application/x-kde-onlyReplaceEmpty" (Klipper hint to prevent clipboard from becoming empty) as an option for pasting.