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

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

examples/demo.zig

@@ -73,11 +73,9 @@ pub fn main() !void {
     };
 
     var container = try App.Container.init(allocator, .{
-        .border = .{
-            .separator = .{ .enabled = true },
-        },
         .layout = .{
             .gap = 2,
+            .separator = .{ .enabled = true },
             .padding = .{ .top = 5, .bottom = 3, .left = 3, .right = 3 },
             .direction = .horizontal,
         },