Improve multi-selection action popup design (with re-using existing UI)

The current UI options make multi-selection cumbersome for end-users. For large dimensions or complex hierarchies, finding and selecting multiple scattered elements with the select element action button or the listbox is difficult because it's easy to lose track of previously selected items while searching for new ones.

Suggested Solution: Replace the current Select Element action popup window with the two-panel interface already used in the View / Ad-hoc Query (left panel displays the full hierarchy/dimension list, right panel displays the selected elements and the search bar is also there at the bottom).

Benefits:

• No new design needed (re-using existing UI) - this request does not require building a new component since the two-panel selection layout already exists and works great in the View.

• Users won't lose their current selection while typing new queries into the search bar.

• Clear visual distinction between available elements and currently selected elements before applying the selection.