Activate theme with parent

[Luc45]

When activating a theme, take into account the scenario of child-parent theme relationship.

• Just one --theme: Activate it
  • If it requires a parent theme, try installing parent from WPORG first, otherwise ask user to provide it
• Two themes: Check if one is parent and another is child, activate the child
• Two unrelated themes: Skip activating
• Three or more themes: Skip activating

[zhongruige]

This tested great for me, thank you @Luc45!

[zhongruige]

Already approved this but went through and tested the latest changes which worked great for me, thanks so much @Luc45!