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

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

examples/elements/scrollable.zig

@@ -73,9 +73,9 @@ pub fn main() !void {
     defer top_box.deinit();
 
     var bottom_box = try App.Container.init(allocator, .{
-        .border = .{ .separator = .{ .enabled = true } },
         .rectangle = .{ .fill = .blue },
         .layout = .{
+            .separator = .{ .enabled = true },
             .direction = .vertical,
             .padding = .vertical(1),
         },
@@ -93,14 +93,12 @@ pub fn main() !void {
     defer bottom_box.deinit();
 
     var container = try App.Container.init(allocator, .{
-        .border = .{
+        .layout = .{
+            .gap = 2,
             .separator = .{
                 .enabled = true,
                 .line = .double,
             },
-        },
-        .layout = .{
-            .gap = 2,
             .padding = .all(5),
             .direction = .vertical,
         },