Make “Browsing Mode” not go in front of all windows

This is generally a better behavior and it helps #50.

Plash/DesktopWindow.swift

@@ -17,7 +17,7 @@ final class DesktopWindow: NSWindow {
 	var isInteractive = false {
 		didSet {
 			if isInteractive {
-				level = .floating
+				level = .desktopIcon
 				makeKeyAndOrderFront(self)
 				ignoresMouseEvents = false
 			} else {

Plash/Menus.swift

@@ -132,6 +132,15 @@ extension AppDelegate {
 			isChecked: isBrowsingMode
 		) { [weak self] _ in
 			self?.isBrowsingMode.toggle()
+
+			SSApp.runOnce(identifier: "activatedBrowsingMode") {
+				DispatchQueue.main.async {
+					NSAlert.showModal(
+						message: "Browsing Mode lets you temporarily interact with the website. For example, to log into an account or scroll to a specific position on the website.",
+						informativeText: "If you don't currently see the website, you might need to hide some windows to reveal the desktop."
+					)
+				}
+			}
 		}
 
 		menu.addSeparator()

Plash/Utilities.swift

@@ -3143,3 +3143,37 @@ extension URL {
 		self.init(string: url)
 	}
 }
+
+
+extension SSApp {
+	/**
+	This is like `SSApp.runOnce()` but let's you have an else-statement too.
+
+	```
+	if SSApp.runOnceShouldRun(identifier: "foo") {
+		// True only the first time and only once.
+	} else {
+
+	}
+	```
+	*/
+	static func runOnceShouldRun(identifier: String) -> Bool {
+		let key = "SS_App_runOnce__\(identifier)"
+
+		guard !UserDefaults.standard.bool(forKey: key) else {
+			return false
+		}
+
+		UserDefaults.standard.set(true, forKey: key)
+		return true
+	}
+
+	/// Run a closure only once ever, even between relaunches of the app.
+	static func runOnce(identifier: String, _ execute: () -> Void) {
+		guard runOnceShouldRun(identifier: identifier) else {
+			return
+		}
+
+		execute()
+	}
+}