Add support for ImGuiIO clipboard callbacks

[scotty007]

Allows user/system clipboard integration (aside from the internal ImGui clipboard and the out-of-the-box supported Windows clipboard).
System clipboard is enabled in GLFW integration.

[coveralls]

Coverage decreased (-0.5%) to 53.607% when pulling be0fe0b on scotty007:feature/clipboard-callbacks into 1d25392 on swistakm:master.

[swistakm]

Thanks for contribution! It features callbacks and we had some memory-safety issues when doing callbacks in different place. So I will just do some testing manually to check whether this needs some warnings in documentation. Will try to do that as soon as possible

[scotty007]

@swistakm Thanks for developing and maintaining this awesome library!

There may also be Python 2/3 type compatibility issues when passing strings around. I can't trigger TypeErrors with the clipboard callbacks, but I stumbled upon such errors when I tried to add support for GetClipboardText() and SetClipboardText(). When I get this solved I can add docstring examples for unit testing (w/o this two functions a would have to fake user input somehow, is this possible?).

BTW: These type errors are triggered in imgui.core.show_style_selector() as well (Python 3). I will open an issue after further investigation.

[swistakm]

Thanks for PR, sorry it took so long to review. I have verified the SDL2 implementation under Python2 and Python3 and fixed the typing issue I think you was referring to.

[swistakm]

Feature released in 1.2.0.

[scotty007]

Thanks for the release! Installed it already :-)