Allow selecting multiple subscriptions for Update and Delete

[fhd]

Background

In the filter preferences, you can currently only update or delete one subscription at a time.

What to change

Make it possible to update or delete several subscriptions at once. The UI for this needs to be discussed first.

[fhd]

Wladimir, you suggested this initially IIRC. Did you have any more concrete thoughts about how this should look UI-wise? If we can figure out the UI beforehand, this would actually be a goodfirstbug IMO.

[trev]

My original thought was allowing multiple selection in the Filter Preferences dialog and then enable only the context menu items that can apply to multiple subscriptions (meaning Update and Delete). Additionally one could use Ctrl-T and Del keys with multiple subscriptions. The Actions buttons should still apply to a single subscription, it doesn't make sense to have them apply to multiple.

However, it probably makes more sense to incorporate these requirements in the preferences redesign.

[Sworddragon]

I have requested this a few years before in the forum: ​https://adblockplus.org/forum/viewtopic.php?f=11&t=9098

Adblock Plus 1.3.x had the same/a similar function but I don't remember anymore how it looked there.

[trev]

Mass-closing all bugs in Adblock Plus for Firefox module, the codebase of Adblock Plus 3.0 belongs into Platform and User-Interface modules. Old bugs are unlikely to still apply.