Link to Mozilla plurals spec
Placeables: Use Translate Toolkit built from repo instead of package

This is necessary since the latest Translate Toolkit package doesn't include
some changes done to placeables code that might be necessary to use.
Placeables: Highlight and copy with mouse click in editor

Fixes bug 3146
Placeables: Add tooltip indicating type of placeable
ALMOST    Placeables: Replace fancy_highlight with hightlight_placeables templatetag
UNDO Add in editor a dialog with keyboard shortcuts list

Fixes bug 2589