Merge pull request #461 from bummzack/master

Fixed an issue with the Vertical- and HorizontalLayout
TiledRowsLayout, TiledColumnsLayout: handling viewPortBoundsbeing null
HorizontalLayout: renamed suggestedBounds to viewPortBounds for consistency with other layouts
ScreenNavigator: fix for case when showScreen() is called when new screen is added to stage, but before transition.
Scroller: removed 'view port wrapper' since it is no longer necessary
Panel: added optional footer. link to wiki.
docs tweaks
PanelScreen: clipping is turned off by default
Item Renderers: fixed icon and accessory positioning with inifinite gap (closes #460)
Slider, ScrollBar, SimpleScrollBar: sub-component factories invalid after direction change because they will need new skins (closes #393)
GroupedList: some inline examples and expanded docs
Header: inline examples and improved docs
docs improvements for sub-component properties
in dispose(), null out properties that added event listeners to external values (closes #464)
extended subComponentProperties docs (and subComponentFactory) to be more clear about what datatype is required (closes #465)
improved docs for sub-component names. added ability to customize the names of scroller scroll bars.
ListCollection: fixed missing Event.CHANGE in removeAll().
DisplayListWatcher: more things are disposed, just in case. mainly just to clear up the memory.
TextInput: added isEditable property (closes #69)
TextInput: better caret position detection (closes #410)