Add Pinch to zoom text in conversation view

This allows to increase/decrease font size in conversation view with a Pinch gesture. Font size also persist during restart.

To test: in conversation view, Pinch to zoom in the middle of the screen. Text size should increase or decrease ( only 4 level allowed ). It persist retween restart

Test click file here: messaging-app.ubports_0.0.0+testing-click-build_armhf.zip messaging-app.ubports_0.0.0+testing-click-build_arm64.zip