gui slider: send SCROLL_CHANGED when value changed

follow the normal slider behaviour, and send EVT_SCROLL_CHANGED when the value is done changing.
This is useful to know the user is done with changing the value (will be used for axis move)

also ensure that we don't hold the mouse capture when disabled.
gui: allow to change the wavelength axis with a slider

introduce new AxisConnector manager, and use it to change the central wavelength of the spectrograph
gui: clean up a bit VigilantAttributeConnector to follow AxisConnector clean up
cli: display the choices meaning if a VA has .choices of type dict

for example, see grating of  spectrograph