Compile only Skia library with INSTALL.md instructions

Update harfbuzz module

Fix syntax error in ASSERT() statement

Fix crash using --save-as CLI

Reported here:
https://community.aseprite.org/t/command-line-save-as-has-stopped-working/1140

file_selector_extras.xml doesn't exist anymore

Fix crash deleting undo states on macOS when running from the app bundle

This bug wasn't reproduced in other situations. Only when running
Aseprite from the app bundle. It looks like app bundles are dynamically
linked with a different free/delete implementation which zeroes/modify
the deallocated memory.

I've posted an answer to this kind of problem on macOS here:
https://stackoverflow.com/a/47817729

Fix X11 app icon

Don't allow shift pixels on locked layer

Discussion: https://community.aseprite.org/t/750

win8: the first touch w/fingers now move the mouse cursor

Use a obs::safe_list() for ui::Timers to avoid data races between thr…

…eads