etc: smaller default position and size of main image window.

Existing default was requesting a window of size 1024×768 at position
(200,100). While it may seem a reasonable default on nowadays displays,
it was not on some intermediate size displays which are considered HiPPI
anyway.

Taking my personal example, my screen is 2560×1440, which is considered
HiPPI by GNOME 3 with a scale ratio of ×2. As a consequence, setting a
size of 1024×768 was actually creating a window of 2048×1536, which is
already higher than the screen. Worse, gtk_window_resize() resize the
window without taking into consideration the title bar, which in my case
added 74 pixels, so GIMP window started at 1610 pixels of height, much
bigger than my screen size, hence unusable (and for some reason, with
the title bar out of the screen so without knowing Super+click shortcut
to move or Super+Up to maximize, people would have a hard time to resize
or close GIMP).

This issue only happens at the first launch of GIMP, when no user
sessionrc exists yet. Once you resize yourself the main window, then
restart GIMP, it is fine (as next times, it will use the user's
sessionrc). Yet it is already a bad first impression.

For temporary workaround, let's use a smaller 800×600 defaults (which
will actually span on 1600×1200 pixels + decoration size on scale ratio
×2).

Still I don't like using arbitrary numbers for window size default.
As we see here, it can end up into all sort of weird result. Even more
with all the scale ratio business which didn't exist back in GTK+2.
Instead, the defaults should have no size, and our code should just
resize to whatever makes the most sense on the current display, which I
believe should likely be maximized. Unfortunately I have a hard time
with gtk_window_maximize() which doesn't seem to do anything at all
(does GNOME ignore _NET_WM_STATE_MAXIMIZE_* hints when requested by
applications maybe?). So until we find the right system, let's go with
this lower window size defaults at least.

etc/sessionrc

@@ -11,8 +11,8 @@
     (size 640 480))
 (session-info "toplevel"
     (factory-entry "gimp-single-image-window")
-    (position 200 100)
+    (position 0 0)
-    (size 1024 768)
+    (size 800 600)
     (open-on-exit)
     (aux-info
         (left-docks-width "188")