Please improve documentation

[ufechner7]

There are no documentation strings. For example:

help?> GtkWindow
search: GtkWindow GtkWindowLeaf @GtkWindow GtkScrolledWindow GtkScrolledWindowLeaf @GtkScrolledWindow

  No documentation found.

  Summary
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  abstract type GtkWindow
...

All types should be documented, in particular how to create an instance, which parameters and keyword parameters are allowed etc.