Rename base/compiler_specific.h to base/override.h
Use a desktop Window in PreviewCommand to avoid a special events loop
Remove rounded corners to minifont.png to make it more readable
Update about dialog
Don't block Menu::showPopup() with a customized event loop

We must call Menu::showPopup() in ui::kMouseUpMessage message.
Rename ui::jmouse_poll to ui::_internal_poll_mouse
Fix crash in ClearCommand removing empty cels
Fix Image::getRowStrideSize() function (fixes crash reported in issue 443)