feat: add PageContainer::getPage

This allows us to directly access a page from outside. This will be
useful for telling the ManagedPackPage who is the window it's on, so
that we can close it when updating :^)

Signed-off-by: flow <flowlnlnln@gmail.com>

launcher/ui/widgets/PageContainer.h

@@ -79,6 +79,7 @@ public:
     }
 
     virtual bool selectPage(QString pageId) override;
+    BasePage* getPage(QString pageId) override;
 
     void refreshContainer() override;
     virtual void setParentContainer(BasePageContainer * container)