panel: replace gtktable with gtkgrid in panel-toplevel.c
panel: don't set panel's initial size to G_MAXINT

Replacing GtkTable with GtkGrid introduced new bug. Warning is -
'Native children wider or taller than 65535 pixels are not supported'.
This is caused by setting panels initial size to G_MAXINT. Changing
initial size to 0 fixes this new bug.