Fix more OS X cursor issues

Fixed problems with a hidden mouse cursor when we activated Aseprite
window from other windows, or from the Mission Control, or with Cmd+Alt.
Remove unused variable
Add support to serialize special unicode chars in ui::Accelerator
Change the way ProgramOptions store specified args

Now we can parse command line arguments sequentially in the same order they
were specified.
Fix UIConext::isUiAvailable() for batch mode
Fix SkinTheme::paintMenuItem() for non AppMenuItem items (e.g. app::NotificationItem)
Add more command line options to export or convert sprites

Added --save-as, --scale, --split-layers, and --import-layer command line
options.
Add --sheet-width and --sheet-height command line options to specify the texture size
Fix formatted output of ProgramOptions when one option has multilines
Add static_assert() to check that Pixman constants match gfx::Region::Overlap
Add gfx/rect_io.h
Add gfx::PackingRects class for packing multiple rectangles in a texture
Add --sheet-pack command line option to improve packing images
Minor fix in CONTRIBUTING.md to clone aseprite repo & submodules
Do not center editor scroll by default when we zoom with keys or zoom tool
Merge branch '1.0'

Conflicts:
	src/app/commands/cmd_sprite_size.cpp
	src/app/document_exporter.cpp