Add clear inventory button

this feature originally allowed clearing of inventory by copying an empty inventory and pasting it over another unit.  This behavior was blocked by https://github.com/SupSuper/OpenXcom/commit/e26ff80137ac7efb1a6f58c67ae0a6704287850f but it was probably for the better, since the feature isn't exactly discoverable.

This adds an explicit clear inventory button, which can be used without affecting the current template.  The default hotkey for the button is 'x'.

This PR also fixes a refresh bug that was evident when cycling through units using the [tab] key while hovering the mouse over a loaded weapon.  the loaded ammo shown now correctly updates when the weapon below the mouse cursor changes.