[feature request] Allow the sliders' values to be set up with number

[trebmuh]

It would be great to be able to set up the sliders' values with entering a number. The workflow could be like that: double click somewhere on the slider and then a pop up box comes in the foreground where we can set up a precise numerical value.
Some talks about it here:
http://sourceforge.net/p/petri-foo/mailman/message/27499896/