Do not clamp aspect by default when using bounds

If the scene item has a bounding box set up for it, do not make it use
aspect ratio clamping by default.

Instead, make it so that shift will turn on aspect ratio, and make it
also apply to all types of bounding box modes.

The only time where aspect ratio clamping should apply by default is
when bounds are not in use (i.e. when a source is just created).  Some
will disagree with me just because that's how photoshop does it, but
we're not photoshop, and I feel that the majority of users will have
more trouble with it disabled by default than enabled by default.

So to sum it up:

If bounds inactive, clamp aspect ratio by default, because scene items
start out with it inactive, and it directly affects the scale.

If bounds active, do not clamp aspect ratio by default, because clamping
to aspect ratio doesn't particularly have an effect for mode bounds
modes except for "stretch to bounds".