Make the list of mods sortable

I'd like to have an easy overview what mods (esp files) are available and which are checked. There could be two checkboxes: [A->Z] and [active]

The alphabetical list allows to see if a mod is selected twice (different datafolders) and which version (if the file name supports it) are available/used. Conflicts - e.g. mod activated twice, different versions activated - could be spotted easier this way. Tests - e.g. if different versions of a mod are compatible with another - can be made easier if scrolling and looking through all entries was history.

(RM-2857 from redmine: created on 2015-08-15 by Who Knows, , relates #2858 (closed))