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).

src/render.zig

@@ -58,10 +58,10 @@ pub const Buffered = struct {
     }
 
     /// Render provided cells at size (anchor and dimension) into the *virtual screen*.
-    pub fn render(this: *@This(), comptime T: type, container: *T) !void {
+    pub fn render(this: *@This(), comptime Container: type, container: *Container, comptime Model: type, model: *const Model) !void {
         const size: Point = container.size;
         const origin: Point = container.origin;
-        const cells: []const Cell = try container.content();
+        const cells: []const Cell = try container.content(model);
 
         if (cells.len == 0) return;
 
@@ -80,7 +80,7 @@ pub const Buffered = struct {
         // free immediately
         container.allocator.free(cells);
 
-        for (container.elements.items) |*element| try this.render(T, element);
+        for (container.elements.items) |*element| try this.render(Container, element, Model, model);
     }
 
     /// Write *virtual screen* to alternate screen (should be called once and last during each render loop iteration in the main loop).