Fix and Improve the Slider

[apantelopoulos]

The slider used to fast-change the values of components from the Properties tab does not work properly when it is used with very small values (e.g. 40nF for a capacitor). This should be fixed.

Also, the usability of the slider could be improved by:

1. When the slider is used, the value in the box gets changed as well (currently only the on-canvas value changes)
2. The slider should start from the midpoint with its original value and can then go (linear) from 0X to 2X this or maybe (log) 1/10X to 10X.