Fix wrong icon in Linux (BL-779)

In release builds we occasionally tried to set the icon before
the handle was created. Due to a mono bug this didn't work.
This change sets the icon after the handle got created.
Seems these comments could be a little more informative, to save people looking up BL-779: something like, "BL-779: a bug in Mono requires us to wait to set Icon until handle created."
Can we also reference a Mono bug report, so we can easily later check whether this is fixed and the workaround can be removed?