add(element): interface for injecting user behavior to containers

Some additional refactoring and documentation updates have also been
applied.

src/app.zig

@@ -27,8 +27,8 @@ const log = std.log.scoped(.app);
 ///       union(enum) {},
 ///   );
 ///   // later on create an `App` instance and start the event loop
-///   var app: App = .{};
-///   try app.start(null);
+///   var app: App = .init;
+///   try app.start();
 ///   defer app.stop() catch unreachable;
 /// ```
 pub fn App(comptime E: type) type {
@@ -38,19 +38,29 @@ pub fn App(comptime E: type) type {
     return struct {
         pub const Event = mergeTaggedUnions(event.SystemEvent, E);
         pub const Container = @import("container.zig").Container(Event);
+        pub const Element = @import("element.zig").Element(Event);
 
-        queue: Queue(Event, 256) = .{},
-        thread: ?std.Thread = null,
-        quit_event: std.Thread.ResetEvent = .{},
+        queue: Queue(Event, 256),
+        thread: ?std.Thread,
+        quit_event: std.Thread.ResetEvent,
         termios: ?std.posix.termios = null,
         attached_handler: bool = false,
-        prev_size: Size = .{ .cols = 0, .rows = 0 },
+        prev_size: Size,
 
         pub const SignalHandler = struct {
             context: *anyopaque,
             callback: *const fn (context: *anyopaque) void,
         };
 
+        pub const init: @This() = .{
+            .queue = .{},
+            .thread = null,
+            .quit_event = .{},
+            .termios = null,
+            .attached_handler = false,
+            .prev_size = .{},
+        };
+
         pub fn start(this: *@This()) !void {
             if (this.thread) |_| return;