mod_seo_sitemap: Fix admin display issue with SEO settings in category

See also #587
mod_base: fix rendering of multiple classes for icon

#fixes 589
mod_base: Fix crash in button scomp when text is not a list

Bug was introduced in #589
mod_admin: flexible display of tabs in connect dialog

When wiring _action_dialog_connect.tpl, the tabs that should be visible
can be set with param tabs_enabled, for instance: tabs_enabled=["new",
"find"]. By default all tabs are enabled (dependent on the context) -
this means no change for existing code.
The first tab in the list is highlighted. The code does not solve the
situation when the first enabled tab is not available in this context.
More work will be needed to enable embed panels, but also here this is
no different than the current code.
A possible future enhancement is to iterate over tabs_enabled to
modularize the code further.
core: Let z_supervisor handle already_started errors gracefully

Add children to the list of managed children when starting them returns
an already_started error.
mod_admin: Show pivot queue size on admin status page, if it's not empty

Also added docs for the new admin_rsc model.
mod_development: Fix another crash

Caused by invalid regex; a context was created for something which was
not a site.