Save user preferences

[atomdmac]

Saves and restores various bits of application state:

• Stroke size
• Stroke opacity
• Stroke color
• Grid visibility and style
• Selected tool

As part of this work, a reusable "Signals" facility has been added via createSignal. This allows multiple functions to subscribe to state changes.

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[lovasoa]

Hey, thanks for the PR! I'm not sure what the use case for this is, though; and I don't think I'd like the brush parameters to be saved. If you have two windows opened, they will be constantly fighting for control over the saved parameters.

[atomdmac]

I actually found it frustrating that I'd lost my settings between refreshes. What if the settings were saved per boars instead of globally?