Add support for setting the mouse cursor image in SDL2

There is now a new function called rut_shell_set_cursor which
temporarily changes the cursor image. This takes an enum of predefined
images. When using the SDL2 backend, this will set the cursor to the
corresponding system cursor.

This function is called by the shell in response to every mouse motion
event when no other handler calls it. The idea is that a widget can
change the cursor whenever the pointer enters its input region and the
shell will then automatically change it back when the pointer leaves
that input region.
split-view: Show a resize cursor when the pointer is over the split

When the mouse pointer is over the split the split view now changes
the cursor to a resize image to make it easier to find the line to
drag.
split-view: Remove some unused constants

It looks like these were left over from a copy-and-paste of the
RutInspector.
split-view: Use a constant to specify the size of the grab area

Previously the size of the grab error was hard coded in the code.
There was a discrepency between the size of the painted area and the
size of the actual input region for the grab. The input region was one
pixel bigger than the area allocated for it so it would end up
overlapping with the area for one of the children. This has been fixed
so that both sizes are now the same.
split-view: Set the child1 transform before the size

The code for handling a size change on the main editor area assumes
that the transform for the area is also correct in order to position
the window-relative viewport. This was not previously correct because
the transform was set after setting the size so it would end up using
the old position for the viewport. This patch just swaps the order so
that it works.