Add an option to keep the normal canvas padding in "show all" mode,
instead of extending the checkerboard pattern indefinitely. This
is useful when wanting to show the image content beyond the canvas,
while still keeping the focus on the canvas; further commits will
extend this mode to behave in more view-related cases as if "show
all" wasn't enabled.
Add a new 'View -> Padding Color -> Keep Padding in "Show All"
Mode" toggle, which controls this behavior, with a corresponding
default-value option in the preferences, under "Image Windows ->
Appearance".
Add a "show canvas boundary" display option, and a corresponding
"View" menu item and default-apperance preferences option. When
enabled (the default), the canvas boundary is shown as an orange/
black dashed line in "show all" mode.
Along with this change, the snap preferences have been moved from
GimpDisplayConfig to GimpDisplayOptions, where it makes much more sense.
One of the consequences is that there is no need to duplicate these
values in GimpDisplayShell anymore to differenciate defaults and
current settings.
In order to make a clear separation between the core modules and the
UI modules, move the necessary enums from display-enums.h and
widgets-enums.h to config-enums.h and the files
gimpdisplayoptions.[ch] from the display to the config module. This
removes the config -> display dependency.
This change has three main benefits
* It lets us remove includes of display files from the config module
* We don't have to link gimp-console and test-config with a subset of
object files from the display module
* It is reflected in devel-docs/gimp-module-dependencies.svg that the
application is made up of core modules and UI modules and that no
core module depends on any UI module
