Do not crash in an empty symmetric IncrementalPlot

This crash could be reproduced by starting QGroundControl, switching to
plot, then clicking the "Symmetric" checkbox.
Merge pull request #392 from johnflux/config

Do not crash in an empty symmetric IncrementalPlot
DataPlot - Don't crash if the user cancels saving the data or image file
CommConfigurationWindow - make it a proper QDialog, and not a floating widget
CommConfigurationWindow - don't try to manually center, now that it is a proper QDialog
CommConfigurationWindow - changing the link type should update the window
Comm Settings - move the "link type" combobox to the top, and clean it up

* 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.
Change QString function parameters to const QString &
LinechartWidget - Make the scaling log button a propper toggle button
Linechart - Make it possible to change time axis range
Improve the UI for editing a text label
Bind support for DSMX with 8 or more channels
MainWindow - remove unnecessary static_cast
Merge pull request #395 from jean-m-cyr/config

Bind support for DSMX with 8 or more channels
Improve the UI for editing the Text Label
Allow user to add a label to a custom widget
Fix changing the dock window title when not in advanced mode
Merge pull request #393 from johnflux/config

Various improvements to DataPlot and Communication configuration window
Fix stupid small compilation mistakes that I made in the last merge
Merge pull request #396 from johnflux/config

Fix stupid small compilation mistakes that I made in the last merge