Merge lp://staging/~sinzui/bzr-gtk/gtksourceview into lp://staging/bzr-gtk

Remove unneeded gtksourceview1 support.

    Launchpad bug: https://bugs.launchpad.net/bugs/672158
    Pre-implementation: no one

bzr crashed with TypeError in apply_gedit_colors() because gedit moved
style and language features to gtksourceview2. bzr-gtk now uses
gtksourceview3. The code to construct style information is not needed.

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RULES

    * Remove unneeded code and tests.
    * ADDENDUM: .modify_font() is deprecated, use override_font instead
      and add a test that it used by the widget.

QA

    * In a tree with changes, run
      bzr gdiff
    * Verify the highlighting matches the colours seen when viewing the
      same diff in gedit.
    * Verify the font is monospace.

IMPLEMENTATION

    * Removed the unneeded calls, then the methods that were called,
      then the imports, then the test.
    * Updated the call to .modify_font to be override_font() because
      the method was deprecated. Wrote a replacement testcase for
      the old test to verify the DiffFileView default construction and
      with gtksourceview.