doc: add @since annotations for API added since 0.3.0

And also add release dates to the NEWS.

We're adding API freely, so this can make life easier for anyone who
wants to stay compatible with an older version.

Signed-off-by: Ran Benita <ran234@gmail.com>
doc: explain keysym/string transformation

The documentation should be clear about what is happening, even if it's
rather unlikely anyone will really dig into the details.

Signed-off-by: Ran Benita <ran234@gmail.com>