lockScreen: Extend modal grab to entire uiGroup
Main.pushModal() used to ensure a compositor grab, but didn't take an input grab. That changed when porting to ClutterGrab, so by limiting the grab to the shield/dialog, it is no longer possible to access top bar menus when the screen is locked.
To fix that, take the grab on the whole uiGroup so that the top bar is included.
Fixes #5470 (closed)