Create a new flag to set Mullvad Browser as default on Linux

To set Mullvad Browser as default using the standalone app, we can do the following:

• Execute ./start-mullvad-browser.desktop --register-app
• Add a %u to the Exec parameter of start-mullvad-browser.desktop as follow: (...) Exec=sh -c '"/home/user/mullvad-browser/Browser/start-mullvad-browser" --detach %u (...)
• Execute xdg-settings set default-web-browser start-mullvad-browser.desktop

It should apparently be relatively easy to create a new --set-default flag to do all of this at once.