Client: Don't allow dropping items while dead

[appgurueu]

Should fix #15728 in ~95%? of cases, effectively going back to the previous state (where the formspec prevented this), and should be something we definitely want to do, no matter whether we fix it fully in the future.

There might technically still be a possible race condition if items are dropped in the very moment the player dies, but that should be relatively unlikely to happen.

Ideally we would also want to disable other controls, but as it turns out that requires different changes in different places and I don't want to pollute this trivial PR.

How to test

As described in the issue. Observe that you simply can't drop things while dead anymore.

[sfan5]

wouldn't it make more sense to put this check in dropSelectedItem

[Zughy]

but that should be relatively unlikely to happen

I can confirm that it happens, although rarely (still annoying)

Ideally we would also want to disable other controls

We should get rid of the death concept and let mods implement it. AUX doesn't work, drop key won't work after this PR, but there are controls that current games (i.e. mine) use whilst dead, like hotbar actions. Changing it will break my game. WASD currently works (so I need to anchor dead players to an invisible entity) but there might be games that would like to exploit such behaviour (e.g. for a ghost state. But that will likely introduce more issues)