Taxonomy Manager: Add a 'Select all' feature.

yes, definitely

agreed. I did see a patch for this function for Drupal 6 in another post ...

duplicated issues #1311616: Mass selection and #968834: "check all" function

...better title for our dashboards ;)

I added two icons to the top of tree for selecting all visible checkboxes and for removing the selection, quite a useful feature.
I took the fastest approach for implementing it, maybe it could be done nicer (both user interface and implementation), so looking for feedback.

You can use a simple script like this
$("form input[type='checkbox']").attr ( "checked" , true );

serjas, your solution would apply if the whole tree was loaded at once, but this is not the case, the different levels are being loaded via AJAX on click. Select all is actually a very complex feature that needs to launch all these AJAX requests, then select all.

The following snippet in console unfolds the tree in full if run several times (as many as max tree depth)

jQuery("#taxonomy-manager-tree .expandable > .hitArea, #taxonomy-manager-tree .lastExpandable > .hitArea").trigger('click');

Then you can run your snippet (adapted):

jQuery("#taxonomy-manager-tree input[type='checkbox']").attr("checked", true);