view: Add trackpad/touchscreen zoom support

Using the new continuous zoom feature, implement pinch to zoom gesture.
Tested on both a trackpad and a touchscreen.

Fixes #5.
1 job for touch-zoom in 1 minute and 55 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #1307523
fedora

00:01:55

50.7%