Analytics Segments drop-downs should work like filters in Excel, PowerBI, and Google Data Studio

[#1]

Currently, each segment in Analytics is stand-alone. You can only pick one at a time in each set, and picking one in a superset does not remove excluded subsegments in a subset, allowing the user to select mutually incompatible segment combinations, resulting in no data.

They should instead work like they do in other familiar apps:

 - Select one or multiple values in a dropdown set

 - When one or more values is selected in a dropdown set, where the categories include subsegments shown in another dropdown set, then those subcategories that are excluded by the choice of the supercategory should not be shown, or shown grayed out.

For example, here I have a superset of 3 content area categories, A, B, and C, each with 3 subpages, 1, 2, and 3. When I filter for Content Area and select only Content Areas A & B, then when I look at the dropdown for Page filter, I see only the pages in the A & B categories; the C1, C2, and C3 pages are not available.

[#2]

Another issue with the drop-down UI: I need to caution new users not to click the X on the right of a selection. They typically think that will just clear the filter selection, then are frustrated to find it's been removed from the list.

For a list of more than 5 items, the user needs to scroll up to see the No Filter option if one of the lower items was previously selected. Maybe... 

1. Don't show the X (remove) option on the currently selected segment

2. Keep the No Filter option "sticky" on top when scrolling