[text] Add single-line-mode to ClutterText

[text] Add single-line-mode to ClutterText

Allow using ClutterText as a single line text field. This is useful for
text fields that accept just a single line of contents by default, and
respond to the Enter key press to execute some action.

The :single-line-mode property enables this behaviour inside ClutterText
by clipping and scrolling the contents of the PangoLayout if they do
not fit the allocated width of the Text actor.