Avoid rendering each time a display property's text is changed

This update the pqDoubleEdit class to only call valueChanged() after
the user is done editing the text. Previously, each character the user
typed would cause the valueChanged() signal to be emitted which would
lead to multiple uneccessary renders.

Change-Id: I954a60f56497dde57d789a8d0785ecee375a824e
Merge topic '13887-avoid-spurious-render' into next

1fe63f0 Avoid rendering each time a display property's text is changed