Simplify the interaction with overlays by adding an OverlayManager

[Snuffleupagus]

The current way that the overlays (PasswordPrompt and DocumentProperties) are handled are less than ideal for a number of reasons. Among those are:

• Currently the code that is used to open/close an overlay needs to be duplicated for every one. If we want to add more overlays (e.g. UI for viewing and setting default preferences #3985), this will only get worse over time.
• The way that the current overlays are implemented also means that adding another overlay would require code changes to all the existing ones.
• There is no way to prevent multiple overlays being opened at the same time, which might lead to "interesting" results.

To avoid the issues described above, and to also unify and simplify how overlays work, this PR introduces the OverlayManager.

This PR is thus the one mentioned in #4777 (comment)

/cc @timvandermeij Would you mind reviewing this?

[timvandermeij]

This PR looks really good. Other than the above nits, I couldn't find anything so far that would need improvement. I'll go over this again soon just to be sure that it's all good to go.

[Snuffleupagus]

@timvandermeij Compared to the first version, I've made a number of changes. Sorry about that!

• The first version wasn't really as modular as it could/should have been. Hence I've changed it so that all overlays are no longer required to be placed in the same container. (As an added bonus, this slightly reduced the number of lines added by the PR.)
• A small change which allows an overlay to also force close itself. This was done to ensure a consistent behaviour, and also to prevent any future issues.
• Add/remove the keydown event listener when opening/closing overlays, since always having an event listener attached to the window seems unnecessary.
• Address the review comments from the previous version.

[timvandermeij]

/botio-linux preview

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[timvandermeij]

Nice work!

[Snuffleupagus]

@timvandermeij Thanks for merging!