Rework guages & indicators

[scudderfish]

The current implementation of gauges is too inflexible and needs nuking from orbit.

• An ecu defines a list of OutputChannels
• An OC is connected to an Indicator. This is a View subclass. It is given an area it can draw in. It maintains the min/max/warning/danger values. It does not draw itself
• An indicator owns a Renderer and delegates drawing responsibility to it
• There are many subclasses to Renderer for Text, Numeric, LED, Gauge, Bar, Histogram
• An Indicator can change Renderer at runtime
• (Optional) Additional data sources can throw extra OutputChannels into the mix, such as GPS, accelerometers, magnetometers etc
• (Optional) MSL logging becomes a dump of all available OutputChannels.

This will fix #109