build: Add desktop files and icons to .deb (fixes #3439) (#5277)

etc/linux-desktop/README.md

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+# Desktop Entries
+
+This directory contains files to integrate Syncthing in your desktop environment (DE).
+Specifically this works for DEs that implement the [XDG Desktop Menu Specification][1], which
+is virtually every DE.  
+To add Syncthing to desktop menus for all users, copy the `.desktop` files to
+`/usr/local/share/applications` (root required). To add it for just your user, copy them to `~/.local/share/applications`.  
+To start Syncthing automatically, you have two options: Either you go to the autostart settings of your DE and choose Syncthing or you copy the `syncthing-start.desktop` file to `~/.config/autostart`.  
+For more information refer to the [ArchWiki page on Desktop entries][2]
+
+[1]: https://specifications.freedesktop.org/menu-spec/menu-spec-latest.html
+[2]: https://wiki.archlinux.org/index.php/Desktop_entries

etc/linux-desktop/syncthing-start.desktop

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+[Desktop Entry]
+Name=Start Syncthing
+GenericName=File synchronization
+Comment=Starts the main syncthing process in the background.
+Exec=/usr/bin/syncthing -no-browser
+Icon=syncthing
+Terminal=false
+Type=Application
+Categories=Network;FileTransfer;P2P

etc/linux-desktop/syncthing-ui.desktop

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+[Desktop Entry]
+Name=Syncthing Web UI
+GenericName=File synchronization UI
+Comment="Opens Syncthing's Web UI in the default browser (Syncthing must already be started)."
+Exec=/usr/bin/syncthing -browser-only
+Icon=syncthing
+Terminal=false
+Type=Application
+Categories=Network;FileTransfer;P2P