Switch labels for some theme regions when language is RTL

This issue began life at #226587: Default sidebar region labels are confusing (wrong) for RTL languages.

Themes support RTL languages by loading additional style sheets where text alignment, float, margin, padding, and offset of elements can be overridden to reflect the LTR design as a RTL design. In the default themes, this has the unfortunate side-effect of putting the left sidebar region on the right hand side of the page for RTL users.

Both common sense and Bojhan have pointed out that users understand labels like "left sidebar" and "right sidebar" when thinking about layout more readily than abstract labels like "primary sidebar" and "secondary sidebar" or "first sidebar" and "second sidebar."

Developers of bidirectional or purely RTL themes may also have some "cognitive dissonance" when working in page templates, theme info files and elsewhere. Default themes use hardcoded region names like sidebar_left and sidebar_right. This issue does not address this problem. Template variable names and page region names are being discussed at #226587: Default sidebar region labels are confusing (wrong) for RTL languages.

This issue aims to improve the usability of the block admin UI in bidirectional and RTL contexts.

The proposed solution is to create a new theme settings array named 'bidirectional_regions' which defines comma separated pairs of regions whose labels should be exchanged when a RTL user works in the block admin form.

regions[sidebar_left]       = left sidebar
regions[sidebar_right]      = right sidebar
regions[content]            = content
regions[header]             = header
regions[footer]             = footer
bidirectional_regions[]     = sidebar_left,sidebar_right

Additional pairs of bidirectional regions would look like this:

regions[sidebar_left]       = left sidebar
regions[sidebar_right]      = right sidebar
regions[content]            = content
regions[header_left]        = left header
regions[header_right]       = right header
regions[footer]             = footer
bidirectional_regions[]     = sidebar_left,sidebar_right
bidirectional_regions[]     = header_left,header_right

The stored value of the block's region will remain normalized to LTR languages.

if user's language is LTR:

$element['region'] = array(
  '#type' => 'select',
  '#options' => array(
    'header' => 'header',
    'sidebar_left' => 'left sidebar',
    'sidebar_right' => 'right sidebar',
    etc...
  ),
);

if user's language is RTL:

$element['region'] = array(
  '#type' => 'select',
  '#options' => array(
    'header' => 'header',
    'sidebar_right' => 'left sidebar',
    'sidebar_left' => 'right sidebar',
    etc...
  ),
);

If themers want to use another method to support bidirectional layout (i.e. their theme uses tables for layout), they could also use the values of this 'bidirectional_regions' array in a preprocess function to swap the contents of the regions too. However, this issue is only concerned with the region labels that appear in the block administration UI.

If a themer uses semantic names for her regions, she can also omit the 'bidirectional_regions' settings from her theme info file to prevent the labels from exchanging places.

I have other patches to work on, but I will happily review patches and if I have time after terms-as-fields, I will do my best on this too.