Add Theme.apply_changes() for any theme changes

[darkowlzz]

NOTE: This is based on the ideas discussed in #9 to implement a
migration path to fix issues in the theme definitions of existing themes.
It implements:

Write a function that can be called that will move old directory (probably accept either Path or PathBuf) to a new one that matches the name (see install for how that's done).
-- @mautamu #9 (comment)

#23 added name validation for new themes. This change is for migrating
the existing themes. This change doesn't add the changes repo field as
discussed in #9 but prepares for the structure to implement it.

This introduces a public method apply_changes() on the Theme type
to apply various changes to an existing theme. The theme changes
field will be added later in the Theme struct. This is a preparation
for the theme changes feature.

The first type of change implemented here is the directory rename
change as Theme::apply_change_rename_dir(). It takes an old theme name
and a new theme name and renames the theme directory if found. This is
called by Theme.apply_changes() when the theme name is not the same
after applying the rename change.

Theme.get_name() returns the effectual name of the theme after
applying any rename change. The implementation will change once the
changes fields are added in Theme struct.

[mautamu]

Nice work! Glad to see that we're slowly getting to the 0.2.0 release :).

I have a POC for the remainder, but large pieces are dysfunctional. Might push to my tree in a bit.

Thanks for tackling this!!