window: stop stomping on resize-mode set by external API

window: stop stomping on resize-mode set by external API

commit c3dc0d80f1353ac66882ca3288ca7e9a13c47d6f fixed the behavior of
GtkContainer widgets requesting an IMMEDIATE resize-mode.

However, GtkWindow has been stomping on resize-mode during realize()
since commit addcc64b9cbb2fb1225080075ad3112a3d93d839. The combination
of factors that led to this not being a visible problem during all this
while is uncertain, but this now causes the Shell to continuously try to
relayout its ShellEmbeddedWindow (a GtkWindow subclass).

This commit separates the resize-mode as set internally by GtkWindow
from the one set with the external API, so that GtkWindow only changes
it when it had not been set before by the subclass.

https://bugzilla.gnome.org/show_bug.cgi?id=763650