Don't discard stack when MenuInventory::itemReturn() fails
Add empty() and clear() functions to ItemStack class
Don't allow MenuItemStorage::click() on empty ItemStacks
Merge pull request #1212 from dorkster/fixes

Don't discard stack when MenuInventory::itemReturn() fails
Display NPC tooltips when using touchscreen
For touchscreen devices, first tap will show tooltip for WidgetSlot

Closes #1181

When using a touchscreen, tapping on a WidgetSlot (inventory, stash,
vendor, or power menu) will highlight it in the same fashion as when
using NO_MOUSE. As a result, the tooltip that matches that slot's
contents will be shown. Tapping again will dismiss the tooltip, allowing
regular interaction with the WidgetSlot.