proof of concept impl for inventory copy/paste

Before I flesh out this code and make it "production ready", I just wanted to run the idea by the devs to see if the contribution would be welcome.

This change adds copy and paste buttons to the soldier inventory screen, visible only during the initial loadout configuration (i.e. when accessed from the base craft inventory button or the initial inventory screen when starting a mission).  The 'Paste' button is initially invisible.  When 'Copy' is clicked, the equipment layout for the current unit is remembered.  When another unit is selected and 'Paste' is clicked, the equipment layout from the first unit is applied to the current unit, using items from the ground.  If there aren't enough of any relevant items on the ground, the corresponding slot is left empty, but the template from the first unit is followed as closely as possible with the available items.

The Copy/Paste buttons and functionality are not available during a mission, when TU management would make this feature awkward.

If this idea is a good one, I'll continue by localizing the strings, adding configurable hotkeys, and implementing the other TODO's in the code.