Only check clicking on NPC when not talking to an NPC

Fixes #1260
Merge pull request #1261 from dorkster/fixes

Only check clicking on NPC when not talking to an NPC
Fix regression in 80fbe3f (NPC hover bug)

Closes #1260

It's simpler to check if any of the NPC-related menus are open.
Fix previous item drag data not being cleared

Fixes #1264

- MenuItemStorage needs drag_prev_slot reset when performing the first
"selection click" when using a touchscreen.
- MenuInventory needs drag_prev_src reset when clicking returns an empty
item stack. This can be from a "selection click" with a touch screen or
simply clicking on an empty slot.
Merge pull request #1265 from dorkster/fixes

Fix regression in 80fbe3f (NPC hover bug)