metacity string changes

[Elijah Newren <newren gmail com>]

I've made a few string changes in metacity with various updates to
metacity.schemas.in.  Mostly just comments, clarifications, spacing
fixes, plus a couple of new options (two new double-click-on-titlebar
actions) that aren't exposed in the UI.  Since these are only ever
really seen if someone uses gconf-editor, I doubt this affects
documentation.  It's probably also very low priority for translators.