Viewports Added ImGui::GetMainViewport() as a way to get the bounds and work area of the host display. (#3789, #1542)

Viewports: Backport a few constructs from 'docking' branch.
Viewports: Added ImGuiViewportFlags_IsPlatformWindow, ImGuiViewportFlags_IsPlatformMonitor, ImGuiViewportFlags_OwnedByApp (none of them were in docking branch yet). ImGuiViewportFlags_IsPlatformMonitor is merely there to convey future intent, not yet used.
Reduce uses of io.DisplaySize.
MainMenuBar: Backport work area handling code from 'docking' branch.
Metrics: Backported "Viewports" debug visualizer from 'docking' branch.
Demo: Rework 'Examples->Fullscreen Window'.
Demo: 'Simple Overlay' demo now moves under main menu-bar (if any) using GetMainViewport()'s work area.