Create a theme handler to manage enabling / disabling of themes

We have a moduleHandler class which is used to assist with enabling / disabling modules. Proposing we have a similar mechanism for themes.

theme_enable()
theme_disable() etc... would be converted so we would end up with

Drupal::themeHandler()->disable($theme_list)
....


This would be useful in #1808248: Add a separate module install/uninstall step to the config import process where we would like to override the default behaviour if we are enabling a theme via a config import.