Delete ignored existing items

[p0l0us]

Purpose

As a user, I wish to remove all files which are ignored by Syncthing in my receive-only folder.

This PR adds "Delete Ignored Files" button, which removes all local files which are ignored. Not strictly, but depending on displaying locally ignored files. (Following "Revert Local Changes" feature pattern)

See #9130 for wider context.

Based on #9131 branch.

Testing

Manually on windows machine it works.

Screenshots

Documentation

syncthing/docs#825

[calmh]

Again, I think we have different definitions of "locally ignored" or I am misunderstanding what you're trying to accomplish. Let's start with a plain text description of what you're trying to accomplish, please.

[p0l0us]

Again, I think we have different definitions of "locally ignored" or I am misunderstanding what you're trying to accomplish. Let's start with a plain text description of what you're trying to accomplish, please.

Let's discuss just here: #9131 since this PR is based on #9131.