cmake: misc fixes
- add option to use webengine
- install rclconfig.py & conftree.py to filters/
conftree.py missed an import on https://framagit.org/medoc92/recoll/-/blob/master/src/python/recoll/recoll/conftree.py#L393 (it seems gitlab's syntax highlight is broken here)
Edited by shenlebantongying