event-editor: Demystify the read-only padlock icon

Non-technical users may not realize why they can not edit some events from read-only subscribed remote calendars.

A symbolic padlock icon can mean anything, and does not suffice to make the causes obvious.
This tooltip makes the UI self-documenting, and incidentally improves accessibility.

Here's what it looks like when combined with its sequel !419 (otherwise you just have to imagine the "Cancel" button still being present on the left):