Timer knob should be "unzoomable"

[flandreas]

The interactive knob that appears when hovering over a timer to change the timer interval during simulation should be "unzoomable", i.e should always have the same size independent of the current zoom factor.

Since the user has to grab the knob with the mouse and turn it around the center, the user's move movements become very big with increasing zoom factors.

Especially relevant for the "Analog" project, where more components like "Resistor" use the knob to change their values.