added on_resize method to widgets so their size can be changed after initialization

[CORRUPTOR2037]

No description provided.

[matanox]

Currently Window.set_size has this line of code in it:

self.dispatch_event('on_resize', width, height)

Which means that when called by application code, it may directly steer the flow of control to an application registered callback handler for the on_resize event. This is not trivial for an application expecting its call to set_size to immediately return, whereas instead the next line of application code to execute will be that of its on_resize registered handler (which is a very awkward flow of control, even if applications can be written to safely manage this flow of control, IMHO).

I hope this PR avoids introducing or repeating this kind of unexpected opportunity for confluence of application code.

It's mildly related to #1116 in that the latter explains how Window.dispatch_event works in these regards.

That said, to counter my own, maybe people more immersed in design patterns for event driven applications find this type of confluence natural or beautiful; otherwise to the rest of us, it just entails more conditional logic in callbacks and a rather loopy analysis of the safety when callbacks interact.