Allow switch_theme() to take a single 'stylesheet' argument

[nacin]

After the work in #20103, we can very safely begin to pass around 'stylesheet' everywhere as a unique identity for a theme.

In [21010/trunk/wp-admin/includes/theme.php], we updated delete_theme() to take $stylesheet rather than $template. This was for clarity only; it works the same. But we should allow switch_theme() to take a single argument as well.

switch_theme() would then end up with two function definitions:

function switch_theme( $stylesheet )
function switch_theme( $template, $stylesheet )

​http://unit-tests.trac.wordpress.org/changeset/744

In [21131]:

Allow switch_theme() to take a single $stylesheet argument.

It now effectively has two function definitions:
function switch_theme( $stylesheet )
function switch_theme( $template, $stylesheet )

fixes #21075.

In [21139]:

Use WP_Theme when switching themes. see #21075 for 3.4.