Create visual editors for decluttering cards and templates.

The new 'custom:decluttering-template' card declares a template. It can
be placed in any view of the dashboard and it is only visible in edit
mode. Use the visual editor to conveniently create a template, configure
the card or element, set variables with their default values, and preview
the results.

The existing 'custom:decluttering-card' card now searches for templates
declared by 'custom:decluttering-template' cards in addition to those
in the traditional decluttering_templates dashboard configuration. Use
the visual editor to conveniently pick an existing template defined
elsewhere, set variables, and preview the results.

Fixed possible race conditions when cards are loaded and streamlined
the logic.

Restored previously set styles when element styles are modified.
The element styling behavior is curiously undocumented...?