refactor(widget): tidy up and comment code a bit

[bnjbvr]

I've been exploring the widget API so as to understand how it works, just for the purpose of review. I couldn't help but tweak things here and there; I think there's still a lot to be done before this code is understandable, but here's a small improvement over the status quo already.

cc @toger5

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 6 lines in your changes missing coverage. Please review.

Project coverage is 84.95%. Comparing base (aca83fb) to head (4130157).
Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/widget/machine/mod.rs	91.89%	3 Missing ⚠️
crates/matrix-sdk/src/widget/mod.rs	92.68%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4263      +/-   ##
==========================================
+ Coverage   84.92%   84.95%   +0.03%     
==========================================
  Files         274      274              
  Lines       29805    29798       -7     
==========================================
+ Hits        25311    25314       +3     
+ Misses       4494     4484      -10     

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[andybalaam]

Looks good, thanks!

[andybalaam]

error!("Couldn't deserialize incoming widget message: {error}");