ui: Improvements for replies and edits

[zecakeh]

Each commit should be rather self-explanatory.

The motivation behind the second commit is for the use with Room::restore_composer_draft. When restoring a draft, but the edited/replied-to item is not available locally, that would result in two requests to the homeserver:

1. When restoring the composer state, the client makes a request with Room::event() to get the content of the edited/replied-to event to be able to display it in the UI.
2. When the user sends the message, the client must first call Timeline::{edit/replied_to}_info_from_event_id(), which makes a request to the homeserver, to be able to call Timeline::edit/send_reply() to send the event.

By exposing the content of the edit/replied-to info, the client needs only to make a single homeserver request with Timeline::{edit/replied_to}_info_from_event_id() and can display the edited/replied-to event in the UI with it.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 84.17%. Comparing base (38a18c3) to head (f31a2c5).
Report is 6 commits behind head on main.

Files	Patch %	Lines
crates/matrix-sdk-ui/src/timeline/mod.rs	37.50%	5 Missing ⚠️
crates/matrix-sdk-ui/src/timeline/event_handler.rs	75.00%	2 Missing ⚠️
...tes/matrix-sdk-ui/src/timeline/event_item/local.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3627      +/-   ##
==========================================
- Coverage   84.21%   84.17%   -0.05%     
==========================================
  Files         256      256              
  Lines       26553    26560       +7     
==========================================
- Hits        22362    22356       -6     
- Misses       4191     4204      +13     

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[bnjbvr]

Thanks, makes sense! Ideally I'd like a small rename, but I can do it later if you'd prefer.
(Thanks in particular for the doc update!)