Only update window title if necessary

[1r0b1n0]

Hi,
On Ubuntu 16.04 (with the kde desktop environment from the kde backports ppa), moving the camera makes the taskbar freeze for several minutes (while using 100% CPU).
This happens because :
When the user does move/rotate the camera
-> the pitch/yaw/... properties are updated
-> setWindowModified( true ) is called to notify the qwidget some modifications are pending
-> in setWindowModified() Qt will update the title bar of the rviz window, to add the ' * '
The problem is that kde's task manager has a hard time processing "new window title" events at several hundred times per second, thus freezes.

This pull request fixes the issue on my system

[wjwwood]

code lgtm, but currently pr testing is broken (we're working on it). I'll wait for that to get fixed before merging.

Also, I'd recommend reporting that issue upstream. It shouldn't be so inefficient to update the Window title 😲

[dirk-thomas]

@ros-pull-request-builder retest this please

[dirk-thomas]

@ros-pull-request-builder retest this please