Add plugin symbol plugin_help() which is called by Geany when the plugin should show its documentation (if any). This symbol is optional, plugins can omit it if not needed.
Add a Help button next to the Configure button in the plugin manager dialog to easily open a plugin's documentation if available.

git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@3524 ea778897-0a13-0410-b9d1-a72fbfd435f5