Allow multiple handlers for same mime type and show selector

If multiple handlers are registered for the same mime types, open a selector to choose which one to use.

Original report:

Now that I implemented PDF support in Reader the need for an official way to select which app to use to handle a give mime type is starting to become an issue. Both NC and OC have an 'official' PDF viewer which does its job quite well and which might for one reason or another be preferred by Reader users over the version I implemented (even though Reader supports things like 2-page spread, position saving, night mode and bookmarks which the official/Mozilla viewer lacks). I could just add a personal preference section to allow users to choose which mime types Reader should handle but that is not a good solution in the light of a multitude of apps all handling the same mime type.

NC (and OC) need a mime type handler selector in the lines of the one used in Firefox:

Is something like this in the works?

Start by locating where MIME type handlers are registered and where the viewer chooses a handler; the issue does not name specific files or tests. Done means multiple handlers can coexist for one MIME type and users can select which handler to use through a selector.