Ladybird: Protect some application commands against non-Tab key windows

Currently, the only NSWindow type in the AppKit chrome is the Tab. Once
we have other window types (e.g. Inspector), commands which assume they
are used on a Tab will either crash or behave weirdly.

This changes the createNewTab: command to accept the tab from which the
new tab is created, rather than assuming that tab is the key window. So
if some JS on a page calls window.open() while a non-Tab window is key,
the new tab will be opened within the same tab group.

This also changes closeCurrentTab: to work on any key window. Regardless
of whether the key window is a Tab or some other window, pressing cmd+W
should just close that window.

Ladybird/AppKit/UI/TabController.mm

@@ -166,7 +166,10 @@ enum class IsHistoryNavigation {
 - (void)createNewTab:(id)sender
 {
     auto* delegate = (ApplicationDelegate*)[NSApp delegate];
-    [delegate createNewTab:OptionalNone {}];
+
+    [delegate createNewTab:OptionalNone {}
+                   fromTab:[self tab]
+               activateTab:Web::HTML::ActivateTab::Yes];
 }
 
 - (void)updateNavigationButtonStates