ENYO-3375: improve Palette's "Expand/Collapse All" buttons feature

Related JIRA: https://enyojs.atlassian.net/browse/ENYO-3375

* expanding/collapsing categories must only be applied on filter results
* fix wrong icon state when a category is emptied by the filter
* fix bad state icon appearance under FF
* prevent empty category for "tap" event
* add component description

Enyo-DCO-1.1-Signed-off-by: Vincent HERILIER vincent.herilier@hp.com