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

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

examples/layouts/mixed.zig

@@ -58,8 +58,8 @@ pub fn main() !void {
     }, element);
     for (0..3) |i| {
         var column = try App.Container.init(allocator, .{
-            .border = .{ .separator = .{ .enabled = true } },
             .layout = .{
+                .separator = .{ .enabled = true },
                 .direction = if (i > 0) .vertical else .horizontal,
             },
         }, .{});