Create a new function is_rtl to simplify checking for text direction

[sivel]

Based on a session at WordCamp SF nacin mentioned that a function is_rtl may be a nice addition to simplify checking for text direction and raise the support of RTL support in themes and plugins.

[nacin]

(In [14360]) Introduce is_rtl(). Use it in core. It only becomes defined when locale is loaded, so it's impossible to use it too early. Deprecate the get_bloginfo('text_direction') call. fixes #13206.

[nacin]

Re-open if there are any issues.

is_rtl() is in locale.php, which means it will become defined rather late in the process -- once it actually becomes accurate. get_bloginfo('text_direction') becomes deprecated.

[westi]

(In [14646]) Restore the correct behaviour of bloginfo('text_direction'). Fixes #13360. See #13206.