Trigger controls update by a timer to improve dragging performance

On some plateform( for example my low powered netbook windows 8.1 )
dragging when grib_pi is running is slowed down by controld update
triggered directly by "setCursorLatLon" function.
To improve that, I propose to use a timer. That off course delays
the update by a few milliseconds, but should not be visible even on a
hight powered device
Merge pull request #149 from Ptizef/master

Trigger controls update by a timer to improve dragging performance