make IO context-specific

[pbuckner]

This should fix #137.

Rather than use a single global _io, use a global hash which is keyed by context pointer.

The only data that needs to be stored are the get & set clipboard functions. All other data contained with _IO() is retrieved through the actual imgui-cpp pointer, so no data is lost on switch of context (which yields a new IO).

Sample test file, demonstrating context-specific set and get clipboards functions attached.

clipboard_test.py.gz

[coveralls]

Coverage decreased (-0.02%) to 55.932% when pulling 3bcbc8e on pbuckner:get_io_fix into 480b4de on swistakm:master.

[KinoxKlark]

Looks good to me, thanks! I will have to triple-check that it does not conflict with #192 since context management has become a little bit convoluted but seeing your implementation I don't think they will be any major issue.