avoid having to type the function pointer names manually.
E.g. plugin_add_toolbar_item(...) is expanded to
p_plugin->add_toolbar_item(...).
Note: For now, building on Windows or with Waf needs genapi.py to be
run manually when adding plugin API functions.
git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@3305 ea778897-0a13-0410-b9d1-a72fbfd435f5
