Widgets: piano designer plugin should edit more properties

[pedrolcl]

This comes from SourceForge Ticket 42.

The Qt designer/Creator plugin for the PianoKeybd widget does not show several already declared properties (this is a bug)

* showLabels
* alterations
* labelOrientation
* labelOctave

and should also include some new properties

* labelFont
* startKey

Implementation in r661 or 7a67ca3 will be available in v2.7.0

[pedrolcl]

Drumstick v2.7.0 has been released.