Rename MouseRegion

MouseRegion might be confusing in the context of a finger touch UI.

Current proposals for a new name are:

• HitArea
• HitRegion

After careful consideration the decision is:

The strongest contender was TouchArea. However, TouchArea should be more than
what MouseRegion is today - multitouch being the most obvious omission.

At the same time, MouseRegion has support for buttons, which is clearly a
property of a mouse.

So, what we have today is a MouseRegion, and it is a good name. But, we will
have a TouchArea in the future, so to keep consistency MouseRegion will be
renamed MouseArea.