add(widget/Input): widget for user inputs

Accepts optional label and placeholders which are rendered accordingly.
The widget only is interactable if it is active. Hence it needs further
control from the outside scope where it is used (likely a corresponding
layout). This also applies to the inputed value, which needs to be
received from the within the owning layout to be used for further
processing (i.e. in custom user events, etc.).

src/app.zig

@@ -296,7 +296,6 @@ pub fn App(comptime E: type, comptime R: fn (comptime bool) type, comptime fulls
                                         };
                                         this.postEvent(.{ .key = key });
                                     },
-
                                     'I' => this.postEvent(.{ .focus = true }),
                                     'O' => this.postEvent(.{ .focus = false }),
                                     // 'M', 'm' => return parseMouse(sequence), // TODO: parse mouse inputs