feat(event): add focus in/out event to SystemEvents

feat(app): add minimal size argument for App.start

Read more corresponding inputs from stdin and convert them correctly
(i.e. in band window resizing), further keys (arrow keys, F-keys, etc.).
Respect the provided minimal size for the application which posts an
error message in case the size is smaller than the requested minimal
size.

examples/padding.zig

@@ -80,7 +80,7 @@ pub fn main() !void {
     });
     defer layout.deinit();
 
-    try app.start();
+    try app.start(null);
     defer app.stop() catch unreachable;
 
     // App.Event loop
@@ -102,6 +102,7 @@ pub fn main() !void {
             .err => |err| {
                 log.err("Received {any} with message: {s}", .{ err.err, err.msg });
             },
+            else => {},
         }
         const events = try layout.handle(event);
         for (events.items) |e| {