[gtksourceview] (6 commits) Non-fast-forward update to branch wip/gutter-style

[Sébastien Wilmet <swilmet src gnome org>]

The branch 'wip/gutter-style' was changed in a way that was not a fast-forward update.
NOTE: This may cause problems for people pulling from the branch. For more information,
please see:

 https://wiki.gnome.org/Git/Help/NonFastForward

Commits removed from the branch:

  c90099e... Various small code clean-ups
  3fc1f31... classic style scheme: set line-numbers background color
  7615754... Apply line-numbers style in GtkSourceGutter

Commits added to the branch:

  75e152e... docs: add (nullable) annotations to many return values (*)
  9b3a138... Adapt to Gtk method signature change (*)
  fcb893c... Replace gtk_source_encoding_foreach() by get_all() (*)
  b51c1a3... Various small code clean-ups
  b8a5da4... classic style scheme: set line-numbers background color
  a1b91bb... Apply line-numbers style in GtkSourceGutter

(*) This commit already existed in another branch; no separate mail sent