Edit unsent messages

[bwindels]

With relations to unsent events supported in matrix-org/matrix-js-sdk#947, this allows editing unsent messages in the UX.

First, it considers pendingEvents while arrow-upping/downing between events to edit.

Second, it does some hackery to allow an uninterrupted experience when the remote echo arrives. When editing an unsent event and the remote echo arrives, the editor will get unmounted and mounted again under a new event tile (with a new key=eventId).

The editor state (current editor model and caret position) needs to be transferred from one editor instances to the new to allow uninterrupted editing. As the editor model depends on the messageeditor react component to be able to trigger autocomplete, we're serializing the model and deserializing it again in the new component.

To keep this state around while the editor is torn down, a new class, EditorStateTransfer, is created, which is passed down from the TimelinePanel all the way down to the MessageEditor, and only replaced when the edit_event action (which is what triggers an edit) is received.

The local echo of editing an unsent event doesn't work yet, as the whole relations infrastructure is implemented in the timelineset, and not present in the pendingEvents... Once the remote echo for the edit arrives, everything falls into place though.

Implements element-hq/element-web#9860
Fixes element-hq/element-web#9864
Needs matrix-org/matrix-js-sdk#947

[bwindels]

Karma tests are failing for reasons unrelated to this PR. Will have a look to fix them in a different PR.

[dbkr]

lgtm - only thing I wondered about was why have a separate class for transferring state between editors rather than just passing the model between them.

[bwindels]

lgtm - only thing I wondered about was why have a separate class for transferring state between editors rather than just passing the model between them.

Because things like PillCandidatePart have indirect references to the MessageEditor react component to open the auto completion, hence the (de)serialization. Also, we need to pass the caret. And I didn't want to have to pass two props all the way down from TimelinePanel to MessageEditor so I grouped it in a new class.