Added warning before bookmark reset

[necocat0918]

cc @bdach

[bdach]

Kinda not sure we want this. Yes it's easy to delete all bookmarks, but undo works with that operation. So you can just ctrl-z, rather than have this gated behind a dialog...

@peppy do you have an opinion here?

[necocat0918]

I thought undo isn't possible for bookmarks, you can close this if you don't want this

[peppy]

I think it's fine to have this, even if it's undoable. Should be used so rarely that the overhead of confirmation is fairly low.

[peppy]

Some unrelated follow-up work I'd see eventually happening around bookmarks:

• Menu items should be disabled when there are no bookmarks to delete / reset
• Fix proximity deletion as discussed in a recent issue thread

This seems fine for now though 👍

[peppy]

@necocat0918 please don't merge master into your PR, it's unnecessary. also please choose better branch names if you're contributing again.