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

This function is called automatically when a QSettings object is destroyed,
so is rarely needed to be called explicitly.

There are still some cases where
a QSettings object is passed-by-reference, so there I left the sync() calls as
it's unclear how permanent those objects are and if they'd call sync() before
application shutdown

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.

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

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.

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().