Issue title: ui38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5InputText widget can't input characters
 Issue body: I use cinder-imgui in ubuntu 14 to write some toy code. When i pressed keyboard to type characters to ui38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5InputText widgets, the widgets' contents didn't insert any new characters. However, when i pressed the delete key, the characters in widgets could be deleted. Even in the test demo window, i can't insert any words in console examples. The 'WantCaptureKeyboard' and 'WantTextInput' flags in test demo windows were set to 1 when i focus and ui38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5InputText. It works when i test samples in imgui, but it dosen't works when i test in cinder-imgui. Could any one can give some advice how to set cinder-imgui properly?
 Comments: 
 Comment 0: Could this be related to https://discourse.libcinder.org/t/cant-type-in-the-imgui-sample-linux/615 (since you're also on Linux)? Or maybe not...

I'm using macOS and finding lots of weird things with the InputText (Multiline) widget.  It often loses shift, control or command key modifiers, subsequently acting like the key remains held down (loses the keyUp?).  Also, I think cut and paste are supposed to work with ImGui, but I've not had any luck with them in Cinder-ImGui.

{'source': 'GithubIssues', 'repo_name_with_owner': 'simongeilfus/Cinder-ImGui', 'issue_id': 290181868, 'repo_stars': 159.0}