shell: Show the submenu icon hint for the applications panel

The panel swarms details/devices/privacy show this icon. Since the applications
panel is not made from sub-panels, this doesn't show. For consistency this shows
the icon so the transition isn't as jarring for the user.

Add a flat into the panel .desktop file to enable this - other panels might
require this in the future
11 jobs for applications-submenu-hint in 4 minutes and 52 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #513869
build

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  Test
passed #513871
asan

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passed #513870
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  Manual
manual #513876
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armv8
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flatpak
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msan
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ppc64le
manual #513873
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tsan
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ubsan
 
  Review
manual #513878
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review
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