mod(container/element): remove origin: Point argument from Element.content function

Renamed `Container.contents` to `Container.content` to be in line with
the corresponding `Element` function name. This has also been done for
the properties structs used by the `Container`.

src/testing.zig

@@ -48,7 +48,7 @@ pub const Renderer = struct {
     pub fn render(this: *@This(), comptime T: type, container: *const T) !void {
         const size: Point = container.size;
         const origin: Point = container.origin;
-        const cells: []const Cell = try container.contents();
+        const cells: []const Cell = try container.content();
 
         if (cells.len == 0) return;