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

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

examples/layouts/grid.zig

@@ -50,16 +50,16 @@ pub fn main() !void {
     const element = quit_text.element();
 
     var container = try App.Container.init(allocator, .{
-        .border = .{ .separator = .{ .enabled = true } },
         .layout = .{
+            .separator = .{ .enabled = true },
             .padding = .{ .top = 5, .bottom = 3, .left = 3, .right = 3 },
             .direction = .horizontal,
         },
     }, element);
     for (0..3) |_| {
         var column = try App.Container.init(allocator, .{
-            .border = .{ .separator = .{ .enabled = true } },
             .layout = .{
+                .separator = .{ .enabled = true },
                 .direction = .vertical,
             },
         }, .{});