Browser: Add "Close tab" action (Ctrl+W) :^)

Note that this is a little bit unreliable with the keyboard shortcut
since LibGUI can get confused about which Action it's supposed to use
as each Browser::Tab has its own "close tab" action. This will need
to be fixed in LibGUI.

Applications/Browser/Tab.h

@@ -44,6 +44,7 @@ public:
     void did_become_active();
 
     Function<void(String)> on_title_change;
+    Function<void(Tab&)> on_tab_close_request;
 
     const String& title() const { return m_title; }