Touchscreen gestures

## Problem

the compatibility with the librem 5 is very theoric: it's hard to:

- zoom
- change tabs
- drag the selection?
- ~~get the selection menu?~~
- exit full-screen

## Suggested solution

implement support for this https://developer.gnome.org/gtk3/stable/GtkGesture.html

- [ ] change tabs with this https://developer.gnome.org/gtk3/stable/GtkGestureSwipe.html if it doesn't break the normal drawing movement
- [ ] drag with this https://developer.gnome.org/gtk3/stable/GtkGestureDrag.html (same condition)
- ~~[x] get the selection menu~~
- [x] zoom with this https://developer.gnome.org/gtk3/stable/GtkGestureZoom.html (no drawback)
- [ ] rotate with a rotation gesture ?
- [ ] exit full-screen with this https://developer.gnome.org/gtk3/stable/GtkGestureLongPress.html if not already implemented
----

the tablet stylus is the issue #69

No source file or test is named. Start by reviewing the linked GTK Gesture, Swipe, Drag, Zoom, and LongPress documentation, then locate the drawing movement and existing zoom or selection-menu entry points; done means touchscreen gestures support the listed interactions without breaking normal drawing.

Written by the indexing model from the issue text.