feat(model): implement Elm architecture

Now the `App` contains a state which is a user-defined `struct` which
is passed to the `handle` and `contents` callbacks for `Container`'s and
`Element`'s. Built-in `Element`'s shall not access the `App.Model` and
should therefore never cause any side-effects.

User-defined events shall be used to act as *messages* to cause
potential side-effects for the model. This is the reason why only
the `handle` callback has a non-const pointer to the `App.Model`. The
`contents` callback can only access the `App.Model` read-only to use for
generating the *view* (in context of the elm architecture).

examples/styles/palette.zig

@@ -5,7 +5,7 @@ const QuitText = struct {
         return .{ .ptr = this, .vtable = &.{ .content = content } };
     }
 
-    fn content(ctx: *anyopaque, cells: []zterm.Cell, size: zterm.Point) !void {
+    fn content(ctx: *anyopaque, _: *const App.Model, cells: []zterm.Cell, size: zterm.Point) !void {
         _ = ctx;
         assert(cells.len == @as(usize, size.x) * @as(usize, size.y));
 
@@ -32,7 +32,7 @@ pub fn main() !void {
 
     const allocator = gpa.allocator();
 
-    var app: App = .init;
+    var app: App = .init(.{});
     var renderer = zterm.Renderer.Buffered.init(allocator);
     defer renderer.deinit();
 
@@ -73,7 +73,7 @@ pub fn main() !void {
             else => {},
         }
 
-        container.handle(event) catch |err| app.postEvent(.{
+        container.handle(&app.model, event) catch |err| app.postEvent(.{
             .err = .{
                 .err = err,
                 .msg = "Container Event handling failed",
@@ -88,7 +88,7 @@ pub fn main() !void {
 
         container.resize(try renderer.resize());
         container.reposition(.{});
-        try renderer.render(@TypeOf(container), &container);
+        try renderer.render(@TypeOf(container), &container, App.Model, &app.model);
         try renderer.flush();
     }
 }
@@ -99,4 +99,4 @@ const log = std.log.scoped(.default);
 const std = @import("std");
 const assert = std.debug.assert;
 const zterm = @import("zterm");
-const App = zterm.App(union(enum) {});
+const App = zterm.App(struct {}, union(enum) {});