WIP: start on refactoring context and CanvasBackend
Refactored example of embedding Canvas via .ui
Finish refactoring of app, only for qt.
Add missing file
Updated all other backends. Not tested yet
refactor app: corrections
app: refactoring+context
app: various small fixes from reviewing the diff
app: fix pyglet after seeing travis error
app: fix wx
app: fix wx + extra test
app: Backends make their context current. GLContext tests
Arg, SET_current instead or make_current. Also forgot test
small fix in app template
address reviewer comments
app: BaseCanvasBackend sets _backend attr of Canvas

This ensures that the Canvas is ready-to-go asap and we can
do draw/show/etc. from the CanvasBackend's __init__

This should fix the error that we saw on wx
Move context.py from util to gloo
Merge pull request #529 from almarklein/context

WIP: Refactoring context and CanvasBackend