impr: Add hooks to let Views to get notified when they are opened or closed (#2493)

This is a trivial change which adds virtual methods to View, `onOpen()`
and `onClose()`, which are called when the view is opened or closed.

This information is already tracked inside the View, but not exposed via
the API. There is `didWindowJustOpen()` and `didWindowJustClose()`, but
these fetch and then reset the flag, so they can't be used more than
once in a frame (and are sometimes called by the frame, meaning the flag
has already been consumed by the time the View's draw callback gets
called).

The use case here is that I have a View which needs to do some work
every time it's shown.

lib/libimhex/include/hex/ui/view.hpp

@@ -110,6 +110,17 @@ namespace hex {
         void trackViewState();
         void setFocused(bool focused);
 
+    protected:
+        /**
+         * @brief Called when this view is opened (i.e. made visible).
+         */
+        virtual void onOpen() {}
+
+        /**
+         * @brief Called when this view is closed (i.e. made invisible).
+         */
+        virtual void onClose() {}
+
     public:
         class Window;
         class Special;

lib/libimhex/source/ui/view.cpp

@@ -75,9 +75,13 @@ namespace hex {
 
     void View::trackViewState() {
         if (m_windowOpen && !m_prevWindowOpen)
+        {
             this->setWindowJustOpened(true);
-        else if (!m_windowOpen && m_prevWindowOpen)
+            this->onOpen();
+        } else if (!m_windowOpen && m_prevWindowOpen) {
             this->setWindowJustClosed(true);
+            this->onClose();
+        }
         m_prevWindowOpen = m_windowOpen;
     }