timegraph: Add selection and mouse pivot to zoom

[mjeanson]

This patch implements the following logic for the zoom pivot point instead of the current behavior of using the center of the visible area:

• Use the mouse pointer X position, if ctrl+shift are pressed and the zoom action is initiated by a mouse scroll event.

• Use the center point of the current time range selection, only if said point is in the current visible range.

• Use the mouse pointer X position, if ctrl is pressed and the zoom action is initiated by a mouse scroll event.

• Use the center of the visible window.

The mouse and selection conditions can also be disabled through appropriate debug options.

Fixes #20.

Signed-off-by: Michael Jeanson mjeanson@efficios.com

[ghost]

Let's merge this, and extra tests can be built on top of it.