Unify single and multiple selection templates and styles on nuxeo-selectivity

Currently, <nuxeo-selectivity> specifies templates for single a multiple selection templates that are hugely different. Not only these templates use different CSS classes, it is almost impossible to properly map equivalents between the two.

CSS classes in single selection:

• selectivity-single-select
• selectivity-single-select-input
• selectivity-single-result-container

CSS classes in multiple selection:

• selectivity-multiple-input-container
• selectivity-multiple-input
• selectivity-placeholder
• selectivity-clearfix

The goal would be to propose a unification of these templates (as much as possible), making it so that both can share a base template and styles.

Advantages:

• Improved consistency between single and multiple selection UI/UX.
• Improved readability and maintainability.
• Avoids issues like ELEMENTS-1181 and ELEMENTS-1189.

Disadvantages:

• Backports (to 10.10 and older) will be less straightforward.
• Can introduce regressions (specially on edge cases) if not handled and tested thoroughly.