more updates to button code

moved all painting into with_texture_canvas code

removed warning for unused borrow

added id assignment to base test code

#5: changed signature and storage type to signed vs unsigned

updated app image

added get/set for id

added match to system event in button event handler

more updates, now gets sent mouse down and mouse up events

added documentation

small cleanup

moved font cache to the engine, not the text widget

more updates to documentation and example app

updating example

updated to display text