Nav: fixed c licking window decorations (e.g. resize borders) from losing...

Nav: fixed c licking window decorations (e.g. resize borders) from losing focused item when within a child window using ImGuiChildFlags_NavFlattened.

In essence, using ImGuiFocusRequestFlags_RestoreFocusedChild here is a way to reduce changes caused by FocusWindow(), but it could be done more neatly.
See amended "nav_flattened" test.