Document struct which inherits from GeanyDocument. This is to hide
implementation fields from the plugin API, so that we can change
them if necessary.
Add DOCUMENT() macro to convert a GeanyDocument* to a Document*.
Also move UNDO_*, FileEncoding to documentprivate.h.
Move undo_action struct to document.c.


git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@2631 ea778897-0a13-0410-b9d1-a72fbfd435f5