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`.

examples/demo.zig

@@ -13,10 +13,9 @@ const QuitText = struct {
         return .{ .ptr = this, .vtable = &.{ .content = content } };
     }
 
-    pub fn content(ctx: *anyopaque, cells: []zterm.Cell, origin: zterm.Point, size: zterm.Point) !void {
+    pub fn content(ctx: *anyopaque, cells: []zterm.Cell, size: zterm.Point) !void {
         _ = ctx;
         std.debug.assert(cells.len == @as(usize, size.x) * @as(usize, size.y));
-        _ = origin;
 
         const y = 2;
         const x = size.x / 2 -| (text.len / 2);