Preferences shows current zoom instead of preference's default zoom (#10455)

Preferences shows current zoom instead of preference's default zoom (#10455)

- Adds a currentZoom variable which holds the current zoom level.

- The zoom stored in preferences is used as default zoom level (default binding:
  M+0).

- The currentZoom is saved and restored via QSettings.

- Adds LFUN buffer-zoom for (re)setting zoom.