Support multiple reference at the Source and Form View area

The linguist application's Source and Form View shows only one line
where the UI text is coded on the source file, though the same UI
text often shared with other files too.
Translators need to consider every context that the UI text is used in.

The attached patches give:

• Show all referencing source files and lines with tabbed view on the
  Source and Form View.
• Show all referencing source files and line numbers at the tooltip of
  the Message Edit area.
• Support font setting on the Source and Form View.