Icon: deprecate icon_size

pixel_size should be always used now.

Change SMALL_ICON_SIZE to cover the other sizes at settings.ini.

Adds a transformation to keep backwards compatibility for some
time. This will be removed in the future.
Keep updated CellRendererInvoker self.path updated - Fixes #4717

Since CellRendererInvoker can set the path at None if a mouse event
is out of the area of the renderer, can break the use of touch,
if happen after the mouse movement. This patch set the path,
in the point_in_cell_renderer test, for the positive case,
solving the issue.

Signed-off-by: Gonzalo Odiard <godiard@sugarlabs.org>