Use a regular dialog rather than a list dialog when trashing or deleting a single file

Summary:
This patch uses a regular-style dialog instead of a list-style dialog when the user is trying to trash or delete only a single item. It looks a bit nicer than showing a dialog that has a list with only a single item in it.

Also change the string "Trash" to "Move to Trash", to prevent ambiguity in languages where the word can be either a noun or a verb.

Depends on D12420

Test Plan:
Made sure that trash and deletion operations all still work. Here's how it looks:

Delete a single file:

Delete multiple files:

Trash a single file:

Trash multiple files:

Reviewers: Frameworks, Dolphin, elvisangelaccio

Reviewed By: elvisangelaccio

Subscribers: elvisangelaccio, cfeck

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12422