Fix mouse issues on OS X (fix #458)

Now we generate she::Event::MouseEnter/Leave events correctly when the
mouse leaves or enters into the window to show the native cursor again
when necessary.

Close #466
Fix Alt+Mnemonic on Mac OS X
Don't add extra ASEPRITE block to .gif file because Photoshop cannot load them
Fix bug moving cels between layers

The image wasn't being copied correctly when the
source cel's (x,y) was != (0,0)
Add some DocRangeOps.MoveCels tests
Fix crash in SelectAccelerator::KeyField when a Unicode char is pressed

The problem is that Accelerator::toString() wasn't returning valid utf-8
strings for Unicode characters.
Merge branch '1.0'

* In this branch the Alt+mnemonic issues are solved in "she" layer when the
  she::Event is generated (see src/she/alleg4/key_poller.cpp)
* Mouse cursor issues on OS X are solved in src/she/alleg4/mouse_poller.cpp

Conflicts:
	src/app/test_context.h
	src/ui/manager.cpp