Add document mirroring feature for #458 (#482)
Add document mirroring feature for #458 (#482)

Allows for vertical or horizontal flipping of images and vectors.
`ButtonToolbar` changed to `ButtonGroup` for uniform look between rotate and mirror buttons.
Rotate and mirror button icons made fixed width for uniform appearance.
Footer colspan modified to fix missing table border section.
LaserWeb/LaserWeb4,LaserWeb/LaserWeb4,LaserWeb/LaserWeb4