godot support 2.2

[Gibbz]

add on doesnt work on 2.2

[eye776]

To get rid of the infinite loop, in GrassPlugin.gd:161 replace
return "".plus_file( path + '/resources')
with
return path.plus_file('/resources')
To get the plugin to display, in GrassPlugin.gd:252 replace
add_custom_control(CONTAINER_SPATIAL_EDITOR_SIDE, grass_editor)
with
add_control_to_container(CONTAINER_SPATIAL_EDITOR_SIDE, grass_editor)

The plugin will at least start now (tested with official 2.1.2 and 3.0alpha).
It still won't work properly because the API has changed quite a bit, but it should be a start.
I could look into this more when I'll have time.