[hooks/operations] use set_operations for three ops (huge gains for massive imports)

[vreg repo] shouldn't call init_registration in on hook registry bootstrap, already done before

[vreg] we should now reset in init_registration since we don't do it in __init__

[web] use uiprops value to compile css transparently, handlig cache and reloading in debug mode

[webconfig] introduce property sheets. Use them to replace external_resources

[webconfig] cleanup base_url management