Added a proper preferences dialog and implement options for background/foreground color

[astrofrog]

This replaces the current Edit settings by a fully-fledged preferences dialog. Plugins can now register preference panes that add a tab to the Preferences dialog.

This also adds the ability to set the background and foreground color for the built-in viewers. Here's an example:

Remaining to-dos:

• Add changelog entry
• Check that things work properly for the custom viewers
• Add tests for new code added here (including to make sure that existing viewers and new viewers have their colors set correctly)
• Default color setting appears to be broken
• Save settings to disk
• Check that dendrogram viewer still works
• Use weak ref for callbacks (no longer relevant)
• Add preference panes docs

After this pull request:

• Adjust the 3D viewers to also recognize the settings and when they are updated.
• Document how viewers should react to changes in the visual settings
• Add sections in settings
• Properly detect which settings have changed in Preferences window

cc @PennyQ, just for info