first mock test of jQuery
added another mock test with jQuery to sprites, improving testcoverage
added palette snapshot
changed canvas_ui_test to ui_canvas_test
create ui_events_tests, testing palette clicks
minnor fixes on test palette clicks
testing pixel editor clicks
tested pixel editor on color changed event
removing duplicated lines on Preview widget
testing preview widget
improving ui events tests with some color_picker tests
ui event tests for sprite selector
removed unused method Preview.change_panel
testing sprite selector next/previous page events