app: [API] add minimized window mode, change methods to options

The window modes are extended, following microsoft conventions.
We have Fullscreen, Overlapping, Maximized and Minimized.
These modes can be set via options when a new window is creates,
or modified later by calling helper functions like w.Maximize() and w.Center()

The window configuration is automatically updated when a user
modifies the window by dragging or clicking the icons on the window's title-bar,
minimizing or maximizing the window.

Any change, either by the user or the application will emit a ConfigChange event.

This is implemented and tested on Windows only.

API change. the app.Window methods Maximize and Center are replaced with similar
options. For example, to maximize a window use

    w.Option(app.Maximized.Option())

Also, Maximize and Center implementations for X11 and macOS are left for a future
change.

Fixes: https://todo.sr.ht/~eliasnaur/gio/315
Signed-off-by: Jan Kåre Vatne <jkvatne@online.no>

app/os.go

@@ -41,6 +41,8 @@ type Config struct {
 	// CustomRenderer is true when the window content is rendered by the
 	// client.
 	CustomRenderer bool
+	// center is a flag used to center the window. Set by option.
+	center bool
 }
 
 // ConfigEvent is sent whenever the configuration of a Window changes.
@@ -57,8 +59,8 @@ func (c *Config) apply(m unit.Metric, options []Option) {
 type wakeupEvent struct{}
 
 // WindowMode is the window mode (WindowMode.Option sets it).
-//
-// Supported platforms are macOS, X11, Windows, Android and JS.
+// Note that mode can be changed programatically as well as by the user
+// clicking on the minimize/maximize buttons on the window's title bar.
 type WindowMode uint8
 
 const (
@@ -66,20 +68,30 @@ const (
 	Windowed WindowMode = iota
 	// Fullscreen is the full screen window mode.
 	Fullscreen
+	// Minimized is for systems where the window can be minimized to an icon.
+	Minimized
+	// Maximized is for systems where the window can be made to fill the available monitor area.
+	Maximized
 )
 
+// Option changes the mode of a Window.
 func (m WindowMode) Option() Option {
 	return func(_ unit.Metric, cnf *Config) {
 		cnf.Mode = m
 	}
 }
 
+// String returns the mode name.
 func (m WindowMode) String() string {
 	switch m {
 	case Windowed:
 		return "windowed"
 	case Fullscreen:
 		return "fullscreen"
+	case Minimized:
+		return "minimized"
+	case Maximized:
+		return "maximized"
 	}
 	return ""
 }
@@ -165,11 +177,6 @@ type driver interface {
 
 	// Wakeup wakes up the event loop and sends a WakeupEvent.
 	Wakeup()
-
-	// Maximize will make the window as large as possible, but keep the frame decorations.
-	Maximize()
-	// Center will place the window at monitor center.
-	Center()
 }
 
 type windowRendezvous struct {