cylc gui: add toolbar item text.

This adds proper text for toolbar items that is
shown when users have particular Gnome configuration.
cylc gui, cylc graph: rename 'landscape' mode.

This renames the 'landscape' graphing orientation mode to
more understandable 'left-to-right' titles.
Merge pull request #857 from benfitzpatrick/cylc-gui.toolbar-text

cylc graph, cylc gui: add toolbar text, rename 'landscape' mode