This allows you to add XY graph plots of any data.  For example,
x,y position data of a servo.

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

Clean up when exactly we should save settings, and add in checks to make sure we do not do it incorrectly

This fixes a bunch of bugs with setting the title for widgets, deleting
widgets, etc.  It also reduces the amount of code duplication, which is
always a good thing.

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

* The "Simulation" option is now not selectable.
* If the link is a simulation, then you can't change it to a different link type.
* The OPAL option is not shown if support is not compiled in.
* Fix the buddy, tab order and accelerators in the GUI.

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