VTE 2.91 fixes, settings scaffolding, semi-auto mem management

This is a big commit with an assortment of things, which would better be
separated in different commits... Nevertheless, this is what happens when
I am lazy and investigating all kinds of ideas at the same time. This is
a list of changes:

- Changes to make Dwt build with VTE 2.91, which are needed now that
  GNOME 3.14 is hitting the streets.
- Scaffolding for a small key/value settings systems backed by the
  file system, in which each setting is a GObject property, and changed
  in the files cause emission of "notify:setting".
- Command line flag to disable using a header bar at run-time. There is
  no need anymore to choose at build time.
- Command line flags are handled for each activation of the GApplication,
  which makes more sense and allows to override settings for each window
  separately using command line options.