EogScrollView: Add pinch/zoom gesture

GtkGestureZoom is used to handle 2-finger pinch/zoom gestures.
If the gesture is triggered, scrolling is cancelled, and both
zoom/offsets are controlled through the 2-sequences bounding box
and distance.
EogScrollView: add image rotation gesture/signal

A ::rotation-changed signal has been added to EogScrollView, that reports
angle changes happening from within the view in 90 degree steps. A
GtkGestureRotate has been hooked to the widget so two-finger touchscreen
interaction does rotate the image.

In order to avoid unintended angle changes when the delta is too close to
a switch point, there is a "grey area" threshold that makes it harder to
switch forth and back again.

This gesture has been added to the same group than the GtkGestureZoom,
as both are meant to share sequences and state.
EogWindow: Handle EogScrollView rotation-changed signal

This makes sure the undo stack gets properly modified, as it happens when
clicking the "rotate" buttons in the UI.
EogScrollView: Add next/previous image signals

Those are triggered by a pan gesture. If the scrollview is not scrollable
horizontally, and a horizontal pan gesture happens, next/prev signals are
emitted on the view. Vertical panning can still trigger scrolling or DnD.
EogWindow: Handle EogScrollView next/previous image signals
Merge branch 'gestures'

https://bugzilla.gnome.org/show_bug.cgi?id=730656