remove duplicate background def
return None if no value defined for style
nicer formatting
support the 'floating' pseudoclass on the dock area
port the dock area styles to enaml stylesheets
add a keyword-safe module alias
only string join if needed
only string join if needed
remove the toolkit specific style sheets

this removes the predefined qt style sheets in favor of the predefined
enaml style sheets. User defined toolkit stylesheets will still be
supported (but will warn) until Enaml 1.0
remove deprecated dock layout support
remove deprecated dock layout support
only string join if needed
support toolkit-specific style sheets until Enaml 1.0
remove support for deprecated dock layout
add support for new style sheets
add a style registry for use by developers and the dock area
update example for new stylesheets