Replace 9 seperate valueChanged() signals with just 1, and remove the resultant tons of code duplication

This crash could be reproduced by starting QGroundControl, switching to
plot, then clicking the "Symmetric" checkbox.

Refactored QGC startup so that MainWindow::instance() is usable everywhere. This was required to get the styling working properly for ChartPlot.

A few more style updates. Added theme awareness to the toolbar. Also switched the plot zoomer over to use blue instead of red as it's selector coloring to match the standard accent color for QGC.

Curve colors are only displayed when curves are being plotted. Also fixed bug when generating CSS coloring from color.

Further consolidation of Plot code into ChartPlot. Also removed some unused variables from LinechartWidget.cc. Also only active curves are recolored in the IncrementalPlot class.

Moved all shared code for IncrementalPlot and LinechartPlot into a common ChartPlot superclass. Also recoloring the background of both widgets now works when switching between color schemes.

Modified the Linechart widget so that the "Show units" button is under the "Curve Selection" frame, which is what it actually modifies.
Also moved all of the UI that was under the "Curve Selection" to inside of it at the bottom so that the grouping of UI elements is more coherent.

Removed some dead code.
Added more valueChanged() signals to account for every data type. This moves the conversion over into the valueChanged receivers, which while it will require more slot-functions, makes it easier to add more code as any necessary conversion is done internally to the slot and so the signaler doesn't need to know the details.
I also added some more details on the types of units expected by valueChanged().