Time edits not updated when changing audio selection and holding Shift

[nielsm]

Depending on whether the Shift key was held during the last mouse move event received during dragging a selection in the audio display, the time edit boxes might not get updated with the new selection times.
This does not relate to the Shift key's usage for toggling snapping to keyframes, automatic keyframe snapping can be on or off, the behaviour described here is the same.

Some examples:

• Hold Shift, start dragging in audio display, stop dragging but keep mouse button held, release mouse, release Shift: edit times not updated.
• Hold Shift, start dragging, stop dragging but don't release mouse, release Shift, release mouse: edit times not updated.
• Start dragging, stop dragging but don't release mouse, hold Shift, release mouse, release Shift: edit times do get updated.
• Start dragging, press Shift, continue dragging, stop dragging but don't release mouse, release Shift, release mouse: edit times do not get updated.
• Start dragging, press Shift, continue dragging, release Shift, continue dragging, stop dragging and release mouse: edit times do get updated.

The behaviour depends entirely on whether the Shift key was held during the last time the mouse was moved before the mouse button was release.

The edit times should be updated regardless of the Shift state during any point of the drag.

[nielsm]

(In [3972]) Make "audio selection changed" state depend not on a flag bug on whether the values asked to be stored back into the time edit boxes actually changed.
Prevent negative selection start/end times in the audio display.
Fixes #942. Should not be ported back to trunk, due to the audio display rewrite.

[nielsm]

Forgot to set milestone.