Visualize signal strengths by location

[alice-i-cecile]

Co-authored by @Austreelis; this work was split out of #600.

Fixes #81.

To do

• ensure that selection is still compatible with signal overlay
• store colors in color palette
• fine-tune color ramp for better legibility, especially for color blind users (split into Vary signal gradient color by type of signal #603)
• swap to using an Option to control which signal is being visualized
• create a GUI to control which signal is being visualized (split into Add signal type selection UI #604)