feat: Add EventViewClosed event (#2377)

Where one door opens, another one closes (said someone) and in this
situation i mean view, because there's no event for closing a view only
opening one, so please add this :c

### Problem description
Simply adds an **EventViewClosed** to match the pre-existing
**EventViewOpened**. Also fixes the issue where EventViewOpened wouldn't
trigger for windows that were popups (even if they are technically a
View.

### Implementation description
I mirrored the current functionality of EventViewOpened for closing.

---------

Co-authored-by: paxcut <53811119+paxcut@users.noreply.github.com>

lib/libimhex/include/hex/api/events/events_gui.hpp

@@ -9,13 +9,10 @@ namespace hex { class View; }
 
 /* GUI events definitions */
 namespace hex {
-
     /**
-     * @brief Signals a newly opened window
+     * @brief Signals a newly opened view
      *
-     * This event is sent when the window has just been opened and docked by the Window manager.
-     *
-     * FIXME: In the event that a newly created window is already docked, this will not be sent.
+     * This event is sent when the view has just been opened by the Window manager.
      *
      * FIXME: This is currently only used for the introduction tutorial.
      *  If the event's only purpose is this, maybe rename it?
@@ -24,6 +21,15 @@ namespace hex {
      */
     EVENT_DEF(EventViewOpened, View*);
 
+    /**
+     * @brief Signals a newly closed view
+     *
+     * This event is sent when the view has just been closed.
+     *
+     * @param view the closed view reference
+     */
+    EVENT_DEF(EventViewClosed, View*);
+
     /**
      * @brief Signals a change in the DPI scale.
      *