Add a link to the Customizer from the Admin Bar appearance menu

[sabreuse]

If we want the theme customizer to make theme setup and tweaking as smooth as possible, users should have a direct link to from the front end of their sites instead of having to go back through the Admin themes page to find it. The attached patch adds it for users with the Switch Themes or Edit Theme Options caps.

[nacin]

Sounds good to me.

Notes on 20575.diff:

• edit_theme_options is all that is necessary. Thus it can simply be moved to directly below the edit_theme_options/return check.
• We have wp_customize_url() to be used here.
• Any call to wp_get_theme() means reading style.css. In this case, get_stylesheet() is the proper thing to take and pass to wp_customize_url().

[sabreuse]

20575.2.diff is a refresh - thanks for the comments!

[ryan]

In [20666]:

Add a link to the Customizer from the Admin Bar appearance menu. Props sabreuse. fixes #20575

[SergeyBiryukov]

Nitpicking: proper whitespace as per the coding standards (20575.whitespace.patch​).

[nacin]

In [20934]:

Theme Customizer: As customize.php without a theme parameter defaults to the current theme, update wp_customize_url() to make $stylesheet optional and update references for the current theme.

see #19910, #20751, #20575.