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/pages/BasePageContainer.h

@@ -1,10 +1,13 @@
 #pragma once
 
+class BasePage;
+
 class BasePageContainer
 {
 public:
     virtual ~BasePageContainer(){};
     virtual bool selectPage(QString pageId) = 0;
+    virtual BasePage* getPage(QString pageId) { return nullptr; };
     virtual void refreshContainer() = 0;
     virtual bool requestClose() = 0;
 };