Added list style to text attributes, independent from paragraph style

Added list style to text attributes, independent from paragraph style
Added list style definition
Added SetListStyle, ClearListStyle, NumberList, PromoteList to
buffer and control classes
Changed style listbox so double-click applies style instead of
single click. This allows for multiple items to be applicable,
and also in future to edit styles from this listbox without
accidentally applying the style to content.
Added wxRichTextStyleListCtrl which also shows a wxChoice for
selecting the type of style to display.
Style sheets can form a chain to allow inheritance from current
style sheet (not yet supported by style list controls).
Added PushStyleSheet/PopStyleSheet.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@42075 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775