Fix EZP-20300: In cases when left panel is hidden:
 - Hiding left panels separator.
 - Wrapping main content into "maincontent" and "maincontent-design" divs.
 - Preventing unwanted CSS changes in the head.
 - Removing "maincontent-desing" left margin in the head.
Fix EZP-20300: Trying another approach:
 - Hiding left panels separator.
 - Changing "maincolumn" right padding, so the extensions content should fit properly.
Fix EZP-20300:
Still another approach:
 - Leaving left panels separator intact.
 - Changing "maincolumn" left padding as well, so the extensions content should fit properly.
Merge pull request #544 from ezsystems/EZP-20300

Fix EZP-20300: Design regression with new admin design for extensions when left menu is hidden