Better msgctxt for month and weekday names

Better msgctxt for month and weekday names

The current msgctxt string 'GDateTime" lead to the unability to
differentiate between the full and abbreviated name for May.
Therefore, the msgctxt strings have been changed to
  'full month name'
  'abbreviated month name'
  'full weekday name'
  'abbreviated weekday name'
This is a string change, but all translations have been updated
using an sed script.

Bug 629429