Print warning when referencing an unknown effect in post_effect

Closes #1253
Remove "class" variable from Item definitions

This was a relic from the old way we did loot generation. It is no
longer used or needed.
Allow overwriting vectors in Item/Set definitions
Allow overwriting of vectors in Power definitions
Allow overwriting vectors in StatBlock definitions
Merge pull request #1255 from dorkster/fixes

Add warning for missing effects and remove unused variable
Merge pull request #1256 from dorkster/data_override

Allow overwriting vectors in Item, Power, and StatBlock definitions
Fix logic bug that could prevent upgrading powers
Merge pull request #1258 from dorkster/fixes

Fix logic bug that could prevent upgrading powers
Prevent spawn powers placing creatures on non-empty tiles
Merge pull request #1259 from dorkster/fixes

Prevent spawn powers placing creatures on non-empty tiles
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)
Add ALT to keybindings list
Don't hard-code hitbox for loot. Use tile size instead
Update README.engine and distribution/flare.man

These files contained obsolete information in regards to paths used by
the engine.
Update INSTALL.engine for Arch Linux

I've linked to miffe's PKGBUILDs for 0.19, as well as my own PKGBUILDs
for Git.