Set Enum-Properties on Elements by their literal Name

[MaZderMind]

Enum-Properties on Elements such as the audiotestsrcElement can currently only be set via their numeric value:

pipeline.getElementByName("test_src").set("wave", 4);

It would be very helpful to be also able to use the literal Enum-Value:

pipeline.getElementByName("test_src").set("wave", "silence");

[neilcsmith-net]

Agreed! In fact, support for setting properties from String in general is something to look at - eg. #166

[neilcsmith-net]

@MaZderMind can you take a look at #190 and see if it covers what you need.