Ladybird/AppKit: Track the currently active tab

Will be needed to open new tabs from other Ladybird processes from the
active tab.

Ladybird/AppKit/Application/ApplicationDelegate.h

@@ -37,6 +37,9 @@
                                fromTab:(nullable Tab*)tab
                            activateTab:(Web::HTML::ActivateTab)activate_tab;
 
+- (void)setActiveTab:(nonnull Tab*)tab;
+- (nullable Tab*)activeTab;
+
 - (void)removeTab:(nonnull TabController*)controller;
 
 - (WebView::CookieJar&)cookieJar;

Ladybird/AppKit/Application/ApplicationDelegate.mm

@@ -35,6 +35,7 @@
 }
 
 @property (nonatomic, strong) NSMutableArray<TabController*>* managed_tabs;
+@property (nonatomic, strong) Tab* active_tab;
 
 @property (nonatomic, strong) TaskManagerController* task_manager_controller;
 
@@ -119,6 +120,16 @@
     return controller;
 }
 
+- (void)setActiveTab:(Tab*)tab
+{
+    self.active_tab = tab;
+}
+
+- (Tab*)activeTab
+{
+    return self.active_tab;
+}
+
 - (void)removeTab:(TabController*)controller
 {
     [self.managed_tabs removeObject:controller];

Ladybird/AppKit/UI/TabController.mm

@@ -520,10 +520,19 @@ static NSString* const TOOLBAR_TAB_OVERVIEW_IDENTIFIER = @"ToolbarTabOverviewIde
     [self.window makeKeyAndOrderFront:sender];
 
     [self focusLocationToolbarItem];
+
+    auto* delegate = (ApplicationDelegate*)[NSApp delegate];
+    [delegate setActiveTab:[self tab]];
 }
 
 #pragma mark - NSWindowDelegate
 
+- (void)windowDidBecomeMain:(NSNotification*)notification
+{
+    auto* delegate = (ApplicationDelegate*)[NSApp delegate];
+    [delegate setActiveTab:[self tab]];
+}
+
 - (void)windowWillClose:(NSNotification*)notification
 {
     [[self tab] tabWillClose];