Form visibility fixes

[cwensley]

This is a follow up to #2684

• Allow calling Window.Show() multiple times without triggering LoadComplete multiple times.
• Wpf: Ensure form is actually shown if GetPreferredSize() was called first.
• Wpf: Prevent the window from appearing before LoadComplete event by deferring setting SizeToContent = manual.
• Wpf: Don't toggle Application.IsActive manually, check every time it is called as it can get out of sync.
• Wpf: FloatingForm.Visible should be false until it is shown.
• Gtk: Actually focus window when Focus() is called, vs. just presenting it.