Assign some mime types by default

Continuation of this discussion: !172 (comment 107454).

Currently, nothing is being set in the mime type database to use the default applications. This is not a huge deal for the web browser, etc., as there are not many mime types to set. However, the primary advantage of setting a default image viewer, audio player, video player, and text editor is that you can switch the default app for many related mime types at once (e.g., you generally would want all image types to open in the same app). However, for this to work, these mime types need to be assigned to the default application in the first place.

We need some mechanism to automatically use these default applications as the default for appropriate mime types, unless and until the user manually sets a different app as the default. Perhaps a post-install script to query xdg-mime for unset defaults and set them?