More consistent with Qml usage pattern

Also many LinkManager API changes to further isolate all link
management inside LinkManager.

Line chart widget: Auto-force ground time if offset is more than 3 seconds (30% of default view width)

Also converted to new logging category

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

Should fix things.

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).

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.

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

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

Although this should theoretically be a no-op it causes a crash in
Windows when running unit tests. Also parent toolbar in constructor.

Correct order of rotations to get roll, pitch and yaw corresponding to classical aeronautics convention and coherent with Fly view

Waiting for real fix to put back. Issue #1275