Use GtkSourceView for editor instead of GtkTextView
* configure.ac: add detection of GtkSourceView dependency for gnome.
* src/gnome/modest-msg-edit-window.c: use GtkSourceView instead of TextView.
We also set a visible margin of 78 characters (good hint for using
typical standards on writing mails). Also fix priority settings.
* src/gnome/ui/modest-msg-edit-window-ui.xml: remove formatting options.
* src/widgets/modest-gtkhtml-msg-view.c: only show the priority in a separate
line for hildon 2.2.