Fix HIL link reconnect issues due to missing threading flag usage, warn user about missing config and take him to config view if necessary

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>

Because the indexes into the stackedwidget are all wrong

There were two separate actions for the simulation view, leading to
strange behaviour when it was selected.

Fixes Issue #542

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

Added documentation and clarified messages to the user.

This feature can be enabled at compile-time and documentation has been added to the README.

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