ThemeSelector Update Phase2

[rabbitism]

1. ThemeSelector now has a new property: ThemeSource.
2. ThemeToggleButton actually controls ThemeSource if it is not set.
3. Instead of maintaining a nullable boolean as internal state, ThemeToggleButton now just looking for next Theme available in ThemeSource.
4. ThemeToggleButton always respect RequestThemeVariant on initialized. if it is set.
5. Introduce a new Control, ThemeSelector. It behaves like a combobox.

Further fix of #340