Editor: Use insets instead of changing the x offset when editing.

The funky cursor behavior is due to the frame's width being changed on iOS 7. Since we want some horizontal padding, set the edge insets instead of fussing with the frame.
Fixes #273.
Use a constant when setting the insets.
Refs #273
Merge pull request #397 from wordpress-mobile/issues/273

Editor: Use insets instead of changing the x offset when editing.