Avoid focusing default window

[feelamee]

Dear ImGui capture keyboard focus even if no ImGui windows was created at all.
I see this behaviour strange and undesirable.

How to reproduce:

• use any example from examples/
• remove all code responsible for creating windows (usually ImGui::Begin, etc)
• rebuild
  => on app startup ImGui::GetIO().WantCaptureKeyboard == true

If required, I can create small example to reproduce.

Also, I noticed same problem in docking branch with ImGui::DockSpaceOverViewport function. If current solution acceptable I will create similar PR for docking too

[feelamee]

Hm.. I researched a little and noticed that window will be created even without Begin()/End() (just using Button and others). So, my solution is incorrect here.

For me, creating default window looks like a hack, but this is usual behavior, so breaking it is not a way.

If you have any ideas how to properly handle this, I can try to implement. Else we can just move this to Issues.