Introduce mixin to customize selection toolbar appearance and position

A lot of users are feeling confused with the selection actions - because of the duplication and the lack of visual impact of the selection toolbar.

As a first step to solve this usability problem, the bgcolor of the toolbar should respect the var(--nuxeo-primary-color); assuring a higher impact and immediate visual acknowledgement of its appearance.

The action icons and clickable texts should all be #FFF and assume the --primary-text-color and #000 on hover.
The 'Clear' option should be grouped with 'Display Selection'

Other improvement

• Allow mixins for
  • specify bar color
  • define absolute position ELEMENTS-644

Implementation
The following variables were added to themes to allow color customization:

 --nuxeo-selection-toolbar-background: var(--nuxeo-primary-color);
 --nuxeo-selection-toolbar-text: #fff;
 --nuxeo-selection-toolbar-link: #fff;
 --nuxeo-selection-toolbar-link-hover: #000;

Also added the --nuxeo-selection-toolbar mixin for further customization (including positioning).