Because a cached dialog stays parented to the initial part, which might
not be the one used to open it.
This is especially visible now that we support more than one window per
okular process.

For example:
 - User initially opens a dialog from window A
      -> This dialog gets cached as child of A's PageView. This implies that
         clicking on the dialog also brings window A to foreground.
 - User re-opens the dialog from a different window B
      -> The dialog is still a child of A's PageView, therefore clicking on the
         dialog's frame brings A to the foreground, and B to the background.