Modulation Matrix issues and improvements

[ustk]

@Christoph-Hart
Something else not related, the labels in the sliders of the mod matrix are preventing the mouse to set the slider even with a display: none css property.
(I derived my css from the snippet browser example so it might behave the same in there, but if a snippet is needed I’ll get home in a short…)

Here I coloured it in black to show the forbidden zone:

• display:none seems to also hide the search bar...
• It would be nice to have a Delete column with a button (feels weird to click either a source or target and select "no connection")