Support building alert filters before evaluating them

Currently, clicking on a single label's + in the alert list view immediately applies that filter. I would like to be able to delay this evaluation via holding down a key.

For example, I would like to hold ctrl, and click on multiple labels' +, which then appear in the filter, but do not get evaluated until I release ctrl. I can also click the x in the filter bar to remove the label, while still holding ctrl.

Selecting multiple labels with the same name will result in them being merged into a regex. For example, selecting instance="instance1", instance="instance2", and instance="instance3" would result in the "filter preview" (unapplied filter, because the user is still holding ctrl) showing instance=~"instance1|instance2|instance3". I'm unsure of doing this merging immediately or after releasing ctrl. Pros of doing the merging immediately (in the "preview" state) is that you can see the final filter and it looks "cleaner". Cons are if you mis-clicked a label and it was merged with another, you would have to either delete the whole label in "preview" mode, or release ctrl and then modify the label filter.

Negations would behave analogously, but with the user holding ctrl+shift.

Discussion points:

1. Do we want this?
2. Do we like the proposed modifiers? ctrl for positive filtering, ctrl+shift for negation
3. Do we want to merge labels immediately (into their regex match/regex negation forms), or after the user releases the modifier key?

The issue names the alert list view, label + and x controls, and filter bar as entry points; first resolve the modifier-key and regex-merging questions before locating their implementation. Done means multiple positive and negative labels can be staged, edited, merged as agreed, and evaluated only when the modifier is released.