Neaten up constants and move getting settings from env out of the main widget.

[samcunliffe]

Solves

• 133

which is nice from a maintainability point of view.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../cavendish_particle_tracks/_decay_angles_dialog.py	66.66%	1 Missing :warning:
...cavendish_particle_tracks/_magnification_dialog.py	80.00%	1 Missing :warning:

Flag	Coverage Δ
	90.59% <92.30%> (+0.05%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/cavendish_particle_tracks/_main_widget.py	93.25% <100.00%> (-0.10%)	:arrow_down:
src/cavendish_particle_tracks/_settings.py	100.00% <100.00%> (ø)
.../cavendish_particle_tracks/_decay_angles_dialog.py	75.34% <66.66%> (+0.34%)	:arrow_up:
...cavendish_particle_tracks/_magnification_dialog.py	94.44% <80.00%> (+0.05%)	:arrow_up:

[samcunliffe]

Too other layers have proper names, could we add them too?

They would need to live in _main_widget.py I guess:

MEASUREMENTS_LAYER_NAME = "Radii and Lengths"
IMAGE_LAYER_NAME = "Particle Tracks"

Oops! I missed them.

[samcunliffe]

Also solves #56