Display settings options need to be generalized and refactored

[tvrusso]

When I first wrote qDF, I envisioned using only APRS as the display. Later, I generalized the display code to support multiple display options such as Google Earth KML files through a "display manager" interface and abstract interface for developing display types.

When I did the first version, the display settings options were manually hacked into the settings dialog box as a panel for APRS settings. Now that it is possible that multiple display types might be active, this needs to be rethought.

Ideally, part of the display interface should include a method for the display object to return a settings widget, which would then be displayed as is in a settings dialog tab. On closing the settings dialog, each widget would then be used by its respective display to set internal settings. It should not be necessary to hand-craft a settings tab for each display type.