#225, #24: rose edit variable label layout

This implements #24 and #225.

#24 is implemented by adding the `FORMAT_CUSTOM` prefixed-properties in `__init__`.py (overridable at the user config level) with options to switch the custom behaviour on.

#225 is implemented by adding description or help labels below the variable title/name, again with options to switch this on or off. Descriptions are shown automatically.

New menu options (and the old `SHOW_MODE_NO_TITLE` one) have gone to a `Layout Preferences` sub-menu under `Metadata`.

The best way to see the new behaviour is to open the `02-help` app in `demo/rose-config-edit/demo_meta`, go to the "Env section title" page, and experiment with the menu options.

@matthewrmshin, please review.