[Meta] Add support for transiant/child windows.

[kalyankondapally]

Currently, we support having only a root window with no transiant/child windows. We should add support for transiant windows and correctly handle parent child relation.

Dependency: #130.
Blocks: #118 .
SubTasks: #133 , #129, #145

[kalyankondapally]

Items which need to be handled:
1)Ensure we clean up egl window of any deleted rootwindow host on GPU process side.
2)Focus management.

1 and 2 are working now, tested with opening the settings menu and opening a new window.

[kalyankondapally]

@tiagovignatti I have separated the task of positioning child windows w.r.t parent to a separate issue. I would prefer it to be handled separately and keep the changes here minimum so we can focus on core support. Please feel free to assign that to your self.

[tiagovignatti]

ok, fine.

[kalyankondapally]

Closing this as the feature is in, lets open individual bugs as we find them.