Theme mods should be able to be gotten/changed on inactive themes.

[georgestephanis]

Currently, there is no way (short of direct option queries) to get theme mods of an inactive theme. This can be problematic when a user wants to see what changes they had configured on a parent theme, when on a child theme, or when trying to see the other theme mods they had configured elsewhere.

This change moves the bulk of the theme mod functions to be methods on the WP_Theme class -- so if you wp_get_theme() any theme -- active or inactive -- you can view and change its mods.

The attached changeset also converts the existing legacy methods to use the new versions on the current theme.

Related: [4401]

[obenland]

I like it. We could probably even remove the _theme part in the method names. It would be interesting to know what the performance impact looks like.