Regression: fix infinite battery notifications

battery_notify should be set to false once the notification was send.
Regression: a single timer causes the app to crash

timer->next has to be properly initialized to NULL or it will lead to
undefined behavior, crashing the app in many cases..
Add support for debug builds on OSX
Fix layer_add_child / layer_insert_below_sibling for API2.0

layer_add_child:
	- will now re-parent the new layer if required

layer_insert_below_sibling:
	- prevent circular references in the next_sibling chain
	- act as a no-op if layers don't have the same parent
Add support for debug builds on Linux
Update documentation to include debug builds
Add missing graphics_draw_rect()
Update pre-built libraries and sim.zip