Fix desktop icon association in GNOME

[bbusenius]

This brings the Linux system template in line with the AppImage template, which received this fix in beeware/briefcase-linux-appimage-template#10.

Without StartupWMClass in the .desktop file, the application icon does not appear when the program is running in GNOME. This same thing happened to Signal Desktop on my system and when I saw signalapp/Signal-Desktop#7515, I realized we could fix it here too.

This PR adds StartupWMClass={{ cookiecutter.app_name }} to the desktop file template. Testing on Ubuntu 24.04 with GNOME 46 confirmed that {{ cookiecutter.app_name }} (e.g., my-app) works correctly, while {{ cookiecutter.formal_name }} (e.g., My App) does not. I'm not sure why we need app_name instead of formal_name but this works on my system.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Thanks for the fix - definitely an oversight, and explains the odd behavior of system icons not appearing... given I thought we'd fixed that problem :-)

[bbusenius]

No problem! Thanks for the great software! I used it to build personal apps that I use daily. It's the most useful thing ever.