[pgadmin4][Patch]: Feature #2686 Property to switch lower/upper caseauto completion

Attached is the patch for the Feature #2686 Property to switch lower/upper case auto completion. I have added new preferences settings "Keywords in uppercase" if it is set to True then all the keywords are shown in upper case in auto completion.

I have also implemented the second request in the RM to change the selected text to upper/lower case. For that I have added new keyboard shortcut "Toggle case of selected text" in preferences dialog. Default key is "Ctrl + Shift + u".

Please review it. 

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