Add dependency with pixman library
Minor changes in resize_image_tests()
Add TLS support to pixman for Unix-like systems
Fix crash in Editor::drawOneSpriteUnclippedRect() if a temporary surface cannot be created (fix #413)
Add Alt+Shift+G shortcut to toogle the pixel grid visibility (fix #504)
Add "View > Show Pixel Grid" menu option to switch the pixel grid
Fix region_tests compilation (as now it depends on pixman library)
Add keyboard shortcuts customization (close #253)

Changes:
* Add KeyboardShortcutsCommand and window
* Add SelectAccelerator window
* Replace modules/gui.cpp functions with app::KeyboardShortcuts and
  app::Key with the logic to load/save/handle keyboard shortcuts
* Change ui::Accelerator concept: now it represent just one keyboard
  shortcut, not a set of shortcuts
* Remove ui::Accelerator from ui::MenuItem, now the key is associated
  in app::AppMenuItem and it's a app::Key
* Add Command::onGetFriendlyName() to get a user friendly name of the
  command depending on its parameters
Move button to delete shortcuts from SelectAccelerator to KeyItem
Fix comparison between ui::Accelerators
Minor change in KeyItem::restoreKeys() logic
Minor layout changes in SelectAccelerator window
Merge branch '1.0'

Conflicts:
	.gitmodules
	CMakeLists.txt
	src/CMakeLists.txt
	src/app/app.h
	src/app/app_menus.cpp
	src/app/commands/cmd_move_mask.cpp
	src/app/commands/cmd_rotate.cpp
	src/app/modules/gui.cpp
	src/doc/resize_image_tests.cpp
	src/ui/accelerator.cpp
	third_party/CMakeLists.txt