Remove icon backgrounds

[provokateurin]

In many cases the white background can be irritating and creates too much focus on the icon.
I checked the same places on web and there is never a background inserted behind the icons/images, so this just aligns the experience with web.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 27.21%. Comparing base (5915c57) to head (5c6b788).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
...mework/lib/src/widgets/unified_search_results.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2448   +/-   ##
=======================================
  Coverage   27.20%   27.21%           
=======================================
  Files         369      369           
  Lines      136676   136665   -11     
=======================================
  Hits        37189    37189           
+ Misses      99487    99476   -11     

Flag	Coverage Δ		*Carryforward flag
account_repository	99.64% <ø> (ø)
cookie_store	100.00% <ø> (ø)		Carriedforward from 5915c57
dashboard_app	96.05% <100.00%> (ø)
dynamite	31.08% <ø> (ø)		Carriedforward from 5915c57
dynamite_end_to_end_test	61.69% <ø> (ø)		Carriedforward from 5915c57
dynamite_runtime	85.40% <ø> (ø)		Carriedforward from 5915c57
neon_dashboard	96.05% <ø> (ø)		Carriedforward from 5915c57
neon_framework	43.69% <0.00%> (+0.14%)	⬆️
neon_http_client	93.28% <ø> (ø)		Carriedforward from 5915c57
neon_notifications	100.00% <ø> (ø)		Carriedforward from 5915c57
neon_talk	99.45% <ø> (ø)		Carriedforward from 5915c57
nextcloud	23.26% <ø> (ø)		Carriedforward from 5915c57
notifications_app	100.00% <ø> (ø)
sort_box	90.90% <ø> (ø)		Carriedforward from 5915c57
talk_app	99.09% <ø> (ø)

*This pull request uses carry forward flags. Click here to find out more.

Files with missing lines	Coverage Δ
packages/neon_framework/lib/src/widgets/image.dart	76.76% <ø> (+6.39%)	⬆️
...ges/dashboard_app/lib/src/widgets/widget_item.dart	100.00% <100.00%> (ø)
...mework/lib/src/widgets/unified_search_results.dart	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes