Fix clip rect for MenuEnemy background graphics

Noticed this bug with the SDL 2 hardware renderer, where the graphics
appeared to be stretched.
Merge pull request #1209 from dorkster/fixes

Fix clip rect for MenuEnemy background graphics
Don't discard stack when MenuInventory::itemReturn() fails
Add empty() and clear() functions to ItemStack class
Always update avatar graphics when changing equipment (even artifacts)

Fixes #1213

Some artifacts can determine which items can be equipped (e.g. Owl
Figurine lets us equip Mage gear with a base Mental of 1). We need to
update the player graphics when removing such artifacts to reflect the
removal of dependent gear.
Merge pull request #1214 from dorkster/artifact_refresh

Always update avatar graphics when changing equipment (even artifacts)
Don't allow MenuItemStorage::click() on empty ItemStacks
NULL-check image creation in SDLSoftwareRenderDevice::createImage()
Merge pull request #1216 from dorkster/renderer_crash

NULL-check image creation in SDLSoftwareRenderDevice::createImage()
show event tooltips for touchscreen by default
Merge pull request #1217 from igorko/master

show event tooltips for touchscreen by default
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.
Merge pull request #1218 from dorkster/android

Tooltips for touch devices
Fix activating power in slot 12 on touch devices

Closes #1211
Combine unlockClick() and click() in MenuPowers
Don't show mouse tooltips when using NO_MOUSE
Always show upgrade buttons for powers that have upgrades

This better exposes which powers have upgrades (and which ones don't).
If the player doesn't meet the requirements to upgrade, the button is
simply disabled.
Allow upgrading powers with keyboard/joypad

Since upgrade buttons aren't in the tablist (and adding/removing them
dynamically would be troublesome), powers can be upgraded by dropping
them immediately after picking them up. This is identical to activating
items in the player's inventory.