AS Crit View: Compact criticals after removing equipment

[SJuliez]

Fighter units use the crit slot system. Equipment is always shown in the crit view as if there were no gaps but removing e.g. equipment no. 2 in a location will currently leave the second slot empty. This leads to the right-click menu not working for some equipment after removing another equipment.

This PR compacts the crit slots on fighters when equipment is removed (deleted or removed by menu or dragged out) from a location, so that there don't remain empty crit slots between equipment. The change is not visible except for the popup menu now working more consistently.

Edit:
This proved a bit of a rabbit hole. So this PR now also:

• Removes the ability to rear-mount weapons on fighters(!) according to TM p.195 (only AFT-mounted weapons can be rear-facing)
• Makes the preview tab update whenever it is shown instead of for every change; this removes the quite noticeable lag when dragging equipment around between slots
• For fighters, greatly reduces the number of actual refreshes happening when the unit is edited by invoking the refresh at the end of the current GUI action (SwingUtilities.invokeLater) and making sure it happens only once. This also helps reducing input lag
• improves placement of the popup menu on the unallocated equipment list in the crit view

[SJuliez]

Need to update this for equipment dragged between locations