StringFormatter: possibility to revert to default value after sending

[klues]

I needed to send a formatted string via the serial port which is formatted like this:
<mousePosX> <mousePosY> <mouseFn> <key1> <key2> <key3>;

and used the StringFormatter plugin to achieve this format. However I needed a possiblity to send some properties only once, like e.g. "mouseFn", which should trigger an action exactly once. To achieve this, this PR adds the possibilty to reset the ports to the default value, after another value was sent, triggered by the corresponding input port.

for details see updated documentation, which will be added soon to this PR.

[klues]

updated documentation with d62c089

[deinhofer]

👍