Import subscriptions

[Schmiddiii]

Licensing

• I confirm that this is either my code or was released under the terms of a GPLv3-or-later compatible license. Also I agree to release it here under the terms of the GPLv3-or-later.

Description

This adds a new dialog window where subscriptions can be imported from either NewPipe or Youtube. This will not override the subscriptions, it will add missing subscriptions to the list instead.

Error handling is still TODO and the wiki also needs to be updated.

[Schmiddiii]

Maybe a note to translators: @daudix-UFO @albanobattistella @Vistaus @vikdevelop @airon90

There have been some new (~15) strings added that will need translation. Feel free to send them in in a PR in the next week.
If I do not get an PR from anyone, I will write a E-Mail and wait another week before a new release.

And maybe @albanobattistella and @airon90 should look if the other one has already made a PR before sending their own.