Improvements to linking UI

[astrofrog]

This includes a few improvements to the linking user interface based on user feedback :)

The UI still 'jumps around' a little when selecting different things, so it will require a little more work.

[codecov]

Codecov Report

Merging #1998 into master will increase coverage by 0.01%.
The diff coverage is 96.33%.

@@            Coverage Diff             @@
##           master    #1998      +/-   ##
==========================================
+ Coverage   87.76%   87.77%   +0.01%     
==========================================
  Files         248      247       -1     
  Lines       21712    21673      -39     
==========================================
- Hits        19055    19023      -32     
+ Misses       2657     2650       -7

Impacted Files	Coverage Δ
glue/plugins/dendro_viewer/layer_style_editor.py	0% <0%> (ø)	⬆️
glue/viewers/scatter/qt/layer_style_editor.py	92.7% <100%> (ø)	⬆️
...lue/plugins/dendro_viewer/qt/layer_style_editor.py	100% <100%> (ø)	⬆️
glue/app/qt/save_data.py	100% <100%> (ø)	⬆️
glue/plugins/dendro_viewer/qt/options_widget.py	92.85% <100%> (ø)	⬆️
glue/viewers/histogram/qt/options_widget.py	86.66% <100%> (ø)	⬆️
glue/viewers/image/qt/layer_style_editor.py	100% <100%> (ø)	⬆️
glue/viewers/histogram/qt/layer_style_editor.py	100% <100%> (ø)	⬆️
glue/viewers/profile/qt/layer_style_editor.py	100% <100%> (ø)	⬆️
glue/dialogs/link_editor/qt/link_editor.py	98.68% <100%> (+0.27%)	⬆️
... and 16 more

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