[Gtk] fix image default size handling

ImageDescription.WithDefaultSize is used to fit the
image into a default size for some component like
a dialog or button. It should always return an image
with the desired default size to fit the system
design guidelines.
(78f999f follow up)
[Gtk] apply default icon size, only if no size is set

the default icon size depends on its purpose. Let backends
decide to use ImageDescription.WithDefaultSize, or not.
[Gtk3] fix libwebkitgtk-3 import names
fix window size for windows without content

(fixes #336)
Merge remote-tracking branch 'sevoku/fix-gtk-default-icon-sizes'
Merge remote-tracking branch 'sevoku/gtk3-fixes'
Merge remote-tracking branch 'sevoku/fix-336'