Moving methods from MainWindow to QGCApplication so that more things
can run without a window. Helpful for testing.

Subclass of QMessageBox which re-implements the static public
functions. There are two reasons for this:
1) The QMessageBox implementation on OSX does now show the title
string. This leads to message boxes which don't make much sense. So on
OSX we set title to text and text to informative text.
2) If parent is NULL, we set parent to MainWindow::instance. This
prevents message boxes which can go behind MainWindow

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

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

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.

Also changed calculateRegression(...) to return a bool, as that's all it needs.

Fixed a stack overflow bug when using the regression fitting command in logfile plot. Solution involved just allocating on the heap instead of the stack.

Significantly improved plotting, now capturing data in linechart allows limmediate processing / analysis of the whole flight