Using QFileDialog directly on OS X can hang intermittently due to Qt
bug.

I don't think it's needed anywhere in QGC since this has been a bug for a long time.

I was unsure how to update the QwtPlotPrintFilter code, so I just commented it out with a FIXME.

MainWindow::loadDataView has been commented out for 9 months now.
Removed commented out code instead of leaving cruft in.

There's no need to create multiple signals - connect() is fine with
connecting styleChanged(MainWindow::QGC_MAINWINDOW_STYLE)  to a slot
with no parameters.

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.