QoL: chest formspecs

[hlqkj]

Hello, this adds the following "features" to chest_api:

• Ability to rename a chest by hitting 'Enter' while focus is on the 'Label' field of the chest formspec.
• Ability to add a player to/remove them from the access list of a silver chest by hitting 'Enter' while the focus is on the respective field.
• A sanity check requiring that only existent players may be added to silver chests (not adding such requirement for removal, ofc.)
• A few feedback messages to the above commands, mainly to report failures.
• The easyvend error sound to the above commands when one of such failures occur. (*1)
• When double-clicking a name in the shared access list of a silver chest, the 'Remove Player' field is automatically filled with the selected name. (*2)
• Two tooltip elements on the silver chest 'Unshare' buttons, warning the player that by clicking them the whole access list will be erased.

The code should be ready for review.

I tried to isolate the features in separate commits such that, by the magics of git, they could somehow be cherry-picked as needed, may something be unwanted. Otherwise, a squash would be advisable!

Thanks for considering this.

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Notes:

1) It may be a little harsh that when sending the easyvend error sound to the client, the standard formspec 'click' sound is also sent and the two overlap. I could not figure how to prevent the 'click' sound from being sent, if that's possible at all?

2) This commit contains some severe ugliness, but it's a take-it-or-leave-it. Doing it properly would require too much effort (a per-player context) which would not be worth for the little benefit it gives in the game, which is, saving a few keystrokes. Please double-check it. I tried to better explain in the code itself (see the comment).

[BluebirdGreycoat]

Wow, thanks!

Regarding the "More QoL, but at what price" commit: there is a player context already available, but it's cheaper for me to just fix that bit of the code than to muck around with git's cherry-picker.

[hlqkj]

Thank you! (And, perfect fix to the players list ugly commit :))