Manipulate bootstrap collapsible with directives

This PR allows to manipulate bootstrap collapsibles (used in the accordion) in a generic, angular specif way. It add 2 new directives which will replace existing, temporary hacks.

`gaCollapsibleTrigger` allows to show/hide collapsibles from within java code. It takes the existing directive (https://github.com/geoadmin/mf-geoadmin3/blob/master/src/components/CatalogcollapsibleDirective.js) and make it more generic, so it is usable for any collapsible element we want

`gaCollapsibleObserve` allows you to observe the state changes in collapsibles in a generic way. It will replace constructs as found here: https://github.com/geoadmin/mf-geoadmin3/blob/d29261af232fcbd527c14846bb8b57c05a77f240/src/components/featuretree/example/index.html#L207