application, main-toolbar: Rename "app.gear-menu" as "app.preview-menu"

There hasn't been a gear since commit 1d3532dd and 3.15.2, and
the icon was changed again in commit f284aa3c when dropping
the application menu. Therefore, it's better to use a name that
describes the menu's purpose and not how the icon looks.

!83
3 jobs for menu-retirement in 16 minutes and 22 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #203689
flatpak

00:15:47

 
  Review
passed #203690
review

00:00:35

manual #203691
allowed to fail manual
stop_review