PrimaryFlightDisplay: Clear up speed label (now showing airspeed and groundspeed separately to avoid confusion) and add vertical climb label

Otherwise some messages say 'System MAV 0-20' and some say 'System 20'.

Should fix things.

FactTextField, when setting the value from its "text" field, was converting the value to string. This was causing (QML JavaScript) code that uses the value for computation to stop working once the user changed the value.

Moved update canvas signal to perspective load instead of hooking it to the paint event.
Added an "S" as the unit for the number of cells fact (as in 4S).

Also changed to qWarning

Added "Advanced Settings" check box.

Created new ScreenTools exported control for QML code to obtain screen DPI factor to be used for computing font point sizes. This code used to be in the tool bar code and now is available to all QML widgets.
Moved global mouse position tool into ScreenTools as I felt it would be a place to collect these sort of functionality.
Replaced Tree used in the Safety Component from a PNG file to a SVG file as the PNG was not scaling up properly.

Tool bar buttons now use QGCButton as base to inherit hover mode fix.
Made QGCButton.showHightLight into a public property so I could use it as a trigger for repainting the chevron button.

Also realized I don’t need __lastMouseXY

Reduced size of the main tool bar buttons accordingly.
Connect/Disconnect buttons (and link configuration combobox) now use the default height.

Passing parameters QVariantMap across C++ boundary was causing crashes
in Qt due to Qml array bugs. Referencing same Fact* across Qml boundary
in multiple QQuickWidgets caused jScript GC to crash.

Switch QGC firmware upgrader for PX4 to AWS and Travis. Use beta/stable/master for Firmware, map all three items to master for Flow (because there are not beta/stable branches)

Button too small for when it switches to disconnect (multiple links adds a drop down icon to the button).