Register AppImages as URL handlers

This is a proposal to make appimaged and the desktop integration application set up a URL handler for certain AppImages which are able to handle URLs. Such AppImages are e.g. browsers, or the [Red Eclipse](https://github.com/TheAssassin/redeclipse-appimage) one I maintain.

As far as my short research on this goes, it should be fairly easy to implement: Just append a line to `~/.local/share/applications/mimeapps.list`. We'd have to somehow keep track of it, however, but we could do that by adding a comment clearly stating that it's an AppImage related entry, and some way to be able to identify it again as being related to an AppImage.

A problem I found is that this might easily override default URL handlers, and thus might create some clutter. We'd probably have to blacklist certain handlers.

All in all I think it's a really cool feature, and might come in handy for many applications.