Removed logic that allowed servoy.executeMethod upcalls to return a
value, to prevent deadlocks on OSX. Instead implemented support for
sending a JavaScript function from the WebView to the Servoy scripting
layer that can be invoked as callback

Also prevented sending non-JavaScript objects (so host or DOM objects)
as arguments into the upcall, as such objects must always be accessed
from the JavaFX Application Thread

Also fixed that consequtive console.log statements in the WebView with
the exact same message were not propagating through to the logger