Add default helper class: ThemeHelper

- Deprecate LayoutHelper::cssClass()
- Deprecate LayoutHelper::themeSetting()
Move $themeSettings variable setup into ThemeHelper

This is kept for BC and will be remove in the future
Add ThemeHelper::icon()
Move $_icon variable setup into ThemeHelper
Remove deprecated calls to LayoutHelper::cssClass()

To migrate existing theme files:

    find path/to/theme \
	-type f \
	-name "*.ctp" \
	-exec sed -i 's/->Layout->cssClass/->Theme->css/' {} \;
Remove deprecated $_icons view variable

To migrate existing theme files:

    find path/to/theme \
	-type f \
	-name "*.ctp" \
	-exec sed -i "s/\$_icons\[\(.*\)\]/\$this->Theme->icon(\1)/" {} \;