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

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

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.

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.

Change so MainWindow handles its own deletion, and deletion of other UI objects to allow for closing when the MainWindow is closed, regardless of if other windows are open.

Refactored QGC startup so that MainWindow::instance() is usable everywhere. This was required to get the styling working properly for ChartPlot.

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.

The light and dark style sheets are now self-contained instead of only coloring. Both the have been synchronized, though the light style sheet still needs some adjustments.

HIL config improvements, fixed confusion about which dock widget is part of which view, main window behaving now

Segregated the coloring of the UI from the rest of the styling in the CSS, so now style-base.css is loaded before the colorizing CSS files.

Selecting the desired stylesheet now works correctly, including saving settings between application launches.

Reworked the styling UI. Now there are 4 options: light, dark, custom_light, and custom_dark. Things don't quite work right, but the proof-of-concept is done.