Inspector: better default split of CSS nodes/props

Inspector: better default split of CSS nodes/props

Move the default pos of the Paned handle to 400px from the left, i.e.
50% of the default width of the window. The previous position at 300px
from left meant the node treeview was too narrow & could easily result
in the (useful) State column not being visible in the case of many
apps. The properties pane doesn't need to be as big as it was anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=788898