PIVOT-696: Support expansion of tabs in TextPane in the same ways as
with TextArea.

Add new style: tabWidth and property expandTabs.

This also required changing the way we get a hold of the TextPane.Skin
from within the skin view classes.  Also PlainTextSerializer needs the
tab settings.


git-svn-id: https://svn.apache.org/repos/asf/pivot/trunk@1551679 13f79535-47bb-0310-9956-ffa450edef68