FIX: imgui hovering when other widget is focused

[dimateos]

Fixes https://github.com/seanchas116/qtimgui/issues/52

• I took the approach of modifying the isActive() virtual function for only the widget and let the polymorphism do nothing new for windows.
• I also modified the value set to MousePos whenever the wrapper is inactive as mentioned in https://github.com/ocornut/imgui/blob/fd943182bd9fe1d801e721a20a41dcde84be39d0/imgui.cpp#L679

Feel free to do edits to the PR.