Add support for editingFinished() signal in QtPropertyBrowser framework

Currently you have to use valueChanged() signal for property managers which is fired on every key stroke and that is far from optimal in many cases.

For example a string property manager emits the valueChanged signal on each key stroke. It would be much better if editingFinished() signal was exposed, so that you could save the property only on enter press or focus leave of the editor.

So adding support for editingFinished() signal would be convenient for the inplace editors which support the same signal (QLineEdit, QSpinBox etc).