Add square function generator. The generator alternates output between

Add square function generator.  The generator alternates output between
two predefined values each N seconds.  Example:

    events {
        40 duplicate square 0 10 20
    }

alternates output between 0 and 10 each 20 secons, starting at 40-th
second of the experiment execution.

General note: none of the event scheduling mechanisms can be controlled
via the GUI, but only through manual editing of the config file.

Another change while here: do not invoke execSetLinkParams() on events
which are changing only link annotations (width, color).

Bug found by:
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