UI/Qt: Clear pressed tab when opening a tab context menu

`TabBar::mousePressEvent()` sets `m_pressed_tab` before
`TabBar::contextMenuEvent()` is called. If we don't clear it to
nullptr, we continue to think that a mouse button is pressed on a tab,
which suppresses the tab preview-on-hover. Clearing it to nullptr makes
that work again.

Fixes #10544

Author: AtkinsSJ

UI/Qt/TabBar.cpp

@@ -809,6 +809,7 @@ void TabBar::contextMenuEvent(QContextMenuEvent* event)
     if (tab_index < 0)
         return;
 
+    m_pressed_tab = nullptr;
     if (auto* tab = m_tab_widget->tab(tab_index))
         tab->context_menu()->exec(event->globalPos());
 }