Ladybird: Implement a basic Inspector window for the AppKit chrome

This commit includes only fetching the DOM tree from the WebContent
process and displaying it in an NSOutlineView. The displayed tree
includes some basic styling (e.g. colors).

Ladybird/AppKit/UI/InspectorController.mm

@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#import <UI/Inspector.h>
+#import <UI/InspectorController.h>
+#import <UI/LadybirdWebView.h>
+#import <UI/Tab.h>
+
+#if !__has_feature(objc_arc)
+#    error "This project requires ARC"
+#endif
+
+@interface InspectorController () <NSWindowDelegate>
+
+@property (nonatomic, strong) Tab* tab;
+
+@end
+
+@implementation InspectorController
+
+- (instancetype)init:(Tab*)tab
+{
+    if (self = [super init]) {
+        self.tab = tab;
+    }
+
+    return self;
+}
+
+#pragma mark - Private methods
+
+- (Inspector*)inspector
+{
+    return (Inspector*)[self window];
+}
+
+#pragma mark - NSWindowController
+
+- (IBAction)showWindow:(id)sender
+{
+    self.window = [[Inspector alloc] init:self.tab];
+    [self.window setDelegate:self];
+    [self.window makeKeyAndOrderFront:sender];
+
+    [[self inspector] inspect];
+}
+
+#pragma mark - NSWindowDelegate
+
+- (void)windowWillClose:(NSNotification*)notification
+{
+    [self.tab onInspectorClosed];
+}
+
+@end