fix(container): positioning; move separator options to layout struct

Added corresponding test cases for padding, borders and corresponding
seperators.

src/element.zig

@@ -35,10 +35,17 @@ pub fn Element(Event: type) type {
         /// `cells` slice has the size of (`size.cols * size.rows`). The
         /// renderer will know where to place the contents on the screen.
         ///
-        /// This function should only fail with an error if the error is
-        /// non-recoverable (i.e. an allocation error, system error, etc.).
-        /// Otherwise user specific errors should be caught using the `handle`
-        /// function before the rendering of the `Container` happens.
+        /// # Note
+        ///
+        /// - Caller owns `cells` slice and ensures that the size usually by assertion:
+        ///   ```zig
+        ///   std.debug.assert(cells.len == @as(usize, size.cols) * @as(usize, size.rows));
+        ///   ```
+        ///
+        /// - This function should only fail with an error if the error is
+        ///   non-recoverable (i.e. an allocation error, system error, etc.).
+        ///   Otherwise user specific errors should be caught using the `handle`
+        ///   function before the rendering of the `Container` happens.
         pub inline fn content(this: @This(), cells: []Cell, size: Size) !void {
             if (this.vtable.content) |content_fn|
                 try content_fn(this.ptr, cells, size);