Use JList instead of a bunch of JPanels in Panes.

This pull request replaces the JPanel based listings of mod packs, Maps and texture packs with JLists using approciate ListModels and ListCellRenderer.

Functionality isn't changed. Minor changes in behaviour are:

* You can't select the Text within the List anymore, which prevents the accidential selecting of text instead of scrolling the list.
* Quickstarting mod packs works as intended when double clicking somewhere on the whole list entry.
* Building and Filtering the lists is slightly more responsive.