applets/digitalclock: reduce number of context menu configure actions

Two commits that should not be squashed:

    applets/digitalclock: reduce number of context menu configure actions
    
    Right now there are three:
    
    - Adjust Date and Time…
    - Set Time Format…
    - Configure Digital Clock…
    
    They're all very similar, and to some people might even sound the same.
    Hard to figure out what specifically they refer to, reducing the ability
    to quickly parse the menu.
    
    In addition, the first two aren't even needed: links to these KCMs are
    present via buttons on the config window you access with the third one.
    
    Accordingly, remove the first two items.
    
    BUG: 499298
    FIXED-IN: 6.4.0

    applets/digital-clock: add "Switch Time Zone…" button in config window
    
    The config window already has a button to let the user navigate to the
    KCM where they can change their date & time format; let's do the same
    for changing the time zone.

Test plan

Right-click on Digital Clock widget and look at the context menu.

Open the config dialog and look at the buttons.

Screenshots or screen recordings

Before	After

Bugs fixed

BUG: 499298
FIXED-IN: 6.4.0

@teams/usability