BUG: notify the scalar bar widget when text properties change.

The ctkVTKTextPropertyWidget was emitting a textChanged signal when
the label format changed, but no signals were emitted when the color,
opacity, font, or font style were changed. Added the modified
signal on the ctkVTKTextPropertyWidget and pass it through as a
modified signal on the ctkVTKScalarBarWidget so that anyone observing
the modified signal can respond to text property changes.

Slicer4 Issue #3027
ENH: emit unique signals when color, opacity, font, style change.

The scalar bar widget listens for the unique signals from the text property
widget and passes them along as a modified signal.
Merge pull request #311 from naucoin/3027-ScalarBar-Respond-To-Updates-From-Labels-Text-Property-Widget

Notify the scalar bar widget when text properties change.