rose metadata: value hint

Added:
 * config_editor/valuewidget/valueshints.py
 Updated associated scripts:
 *  config_editor/valuewidget/__init__.py
 * rose/variable.py
 * rose/metadata_check.py
Updated documentation to reflect new widget capabilities:
 * __init__.py
 * doc/rose-rug-mtadata.html
 * doc/rose-configuration-metadata.html

Close #1322.
We could make use of [this](http://www.pygtk.org/pygtk2reference/class-gtkentrycompletion.html#method-gtkentrycompletion--set-minimum-key-length) as well to allow all the matches to be shown when there's no text.