Modified TrackLayout and StretchableButton to allow their use with ScrollPane
Added new method in FixedButton and new type of button
Added UI for add buttons (that represent tweens) in TrackLayout
Added javadoc and changed one i18n text
Added some changes to make compatible DraggableScrollPane with StretchableButton
Merge pull request #685 from gorco/AddTweensUI

Added tweens ui