Touchpad gesture support (e.g. for zooming via touchpad pinch gestures)

[Povilas Kanapickas <povilas@xxxxxxxx>]

Hello,

I would like to implement support for touchpad gesture handling to
LibreOffice. This is useful e.g. for zooming views using two-finger
pinch touchpad gesture or rotating objects using two-finger rotate gesture.

I have implemented this functionality in GIMP and would like to do the
same for LibreOffice. In LibreOffice case the implementation will be
more complex, as there are more backends than just GTK, but this will
just require more work.

There is a proof of concept implementation that wires touchpad gestures
on GTK to the ViewShell widget. As a result it's possible to use
touchpad pinch gesture to zoom in-out the main slide in Impress.

https://gerrit.libreoffice.org/c/core/+/138791/1 (note that
implementation is split across 5 changes in the chain).

Please let me know what do you think about this problem and the proof of
concept solution.

I would like to eventually add touchpad gesture support to all UI
elements of LibreOffice where that makes sense.

Cheers,
Povilas