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.

src/event.zig

@@ -17,6 +17,7 @@ pub const SystemEvent = union(enum) {
     err: Error,
     resize: Size,
     key: Key,
+    focus: bool,
 };
 
 pub fn mergeTaggedUnions(comptime A: type, comptime B: type) type {