Workaround appears-as-list bug

Adds a GTK theme workaround for themes that set GtkComboBox's
appears-as-list style property. This property is broken and causes the
application to crash.
(https://bugzilla.gnome.org/show_bug.cgi?id=729651)

Fixes Mantis #1254.
Set 'modified' bit when updating Settings panel

Since the settings of story format, Blorb creation, etc., are stored in
the project settings file, then it makes sense to set the 'modified' bit
when those settings changed, so that the changes will be saved.

Partly fixes Mantis #1248.
Don't always set format to Z8

Fix typo that caused all projects' format to change to Z8.

Fixes Mantis #1248.
Refactor common code

Put the code that shows a dialog box allowing the user to save before
exiting into a helper function.
Gracefully handle display name when no file

If a document has not yet been associated with a title or an on-disk
file, then don't try to query its filename.
Remember state settings

Properly remember the state of settings such as View/Toolbar, Notepad,
etc. These were previously remembered but clobbered when opening a new
story.

Fixes Mantis #1277.