Use descriptive categories for the Linux launcher

[njuneau]

At the moment, the Linux application launcher (resources/linux/app.desktop) uses the Application category. Under launchers with category display such as Plasma desktop's Kickoff, this would land under the "Lost & found" section given that "Application" is an unrecognized category.

This PR changes the Application category in favour of:

• The "Settings" category, which is part of the registered main Freedesktop categories ( ref.: https://specifications.freedesktop.org/menu-spec/latest/apa.html#main-category-registry )
• The "Security" category, which is an additional one that can be used alongside "Settings" ( ref.: https://specifications.freedesktop.org/menu-spec/latest/apas02.html )

Tested under Kickoff, this indeed moves the application launcher from "Lost & Found" to "Settings" (which is a bit more descriptive a category).

[boldsuck]

+1 for not use Application category. Yubikey uses Categories=Utility; I also used this for OnlyKey and other 2FA keys.

[onlykey]

Thanks, I was wanting to make sure the category we go with works with most desktops. We will try using Utility.