Owlkettle 2.0.0 is a declarative GUI framwork for GTK 4

[can-lehmann]

I feel like pragmas generally look a bit out of place in Nim. This might 
however be a good thing in the case of the main language, since they usually 
serve a distinct purpose. For the gui DSL there might be better syntax choices. 
I am open to suggestions here. The main requirement is that a potential new 
syntax has to clearly separate the expand attribute from the fields of both the 
parent and child widget (since expand/resize/shrink/... are not a fields of the 
widgets, but rather attributes of the edge between parent and child in the GUI 
tree).