feat(container): introduce fixed_size property for fixed sizing of Containers

Moved min_size property from `Container` to the `Scrollable` element,
where it is only used anyway.

examples/demo.zig

@@ -51,11 +51,14 @@ pub fn main() !void {
     // - some sort of chat? -> write messages and have them displayed in a scrollable array at the right hand side?
     // - on the left some buttons?
     var box = try App.Container.init(allocator, .{
+        .border = .{
+            .color = .blue,
+            .sides = .all,
+        },
         .layout = .{
             .gap = 1,
             .padding = .vertical(2),
         },
-        .min_size = .{ .cols = 50 },
     }, .{});
     try box.append(try App.Container.init(allocator, .{
         .rectangle = .{ .fill = .light_green },
@@ -70,6 +73,7 @@ pub fn main() !void {
 
     var scrollable: App.Scrollable = .{
         .container = box,
+        .min_size = .{ .cols = 60 },
     };
 
     var container = try App.Container.init(allocator, .{
@@ -87,9 +91,11 @@ pub fn main() !void {
             .color = .light_blue,
             .sides = .all,
         },
+        .fixed_size = .{ .cols = 200 },
     }, .{}));
     try container.append(try App.Container.init(allocator, .{
         .rectangle = .{ .fill = .blue },
+        .fixed_size = .{ .cols = 30 },
     }, .{}));
     defer container.deinit(); // also de-initializes the children