HiPS browsing tree

[bmatthieu3]

This features the development of a new HiPS image browsing tree panel.
The filter is still very minimal, allowing user to only filter by min resolution and bandwidth
The mocserver is queried to know which hips have pixels in the current viewport.

Things to do:

• a erase button to reset the content of the search input text
• a button to hide the hips that have no pixels in the current viewport (i.e. hide the orange underlined hipses)
• button next to each HiPS to only load its MOC, target into its (hips_initial_fov, hips_initial_ra, hips_initial_dec)

[bmatthieu3]

Impl: #3 with a new toolbar object.
There is a aladin.getToolbar method to get the toolbar, a add(name, widget) and remove(name) method on it allow the user to add different actions.