Revert "build: Don't use GLIB_GSETTINGS as it's broken with srcdir != builddir"

This reverts commit fec5954f20afe30d74669917894f41ae68bd5206.
app: Bind Control-Shift-T to win.new-tab

Since that's what gnome-terminal users expect.
gssh: Aovid potential use-after-free

If we return an error, the app may unref the object, so we need to
emit state changes, and free any memory just before doing the task
return.
gssh: Squash more use-after-free

We really need to hold a refcount on the whole object, otherwise the
socket iteration will potentially access freed memory.