Replace the usage of the old Scintilla indicator API by the new modern API and remove old hacks (patch by Jason Oster, thanks).
Add new sci_indic_clear() function to the plugin API.

git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@2966 ea778897-0a13-0410-b9d1-a72fbfd435f5