When set as default browser on Linux in standard mode, links don't open correctly

@boklm It turns out our fix from #314 (closed) is incomplete.

While the browser is listed in the default browser lists. It doesn't open correctly when clicking on a external link from another applications (at least in some circumstances).

This is because in the /usr/share/applications/mullvad-browser.desktop file, for the entry Exec, there should be an additional %U parameter to forward correctly the clicked link.

The full desktop file would read as follow:

[Desktop Entry]
Version=1.0
Type=Application
Name=Mullvad Browser
GenericName=Web Browser
Comment=Mullvad Browser is a privacy-focused web browser designed to minimize tracking and fingerprinting.
Categories=Network;WebBrowser;
Exec=/usr/lib/mullvad-browser/start-mullvad-browser --detach %U
Icon=mullvad-browser
StartupWMClass=Mullvad Browser
MimeType=text/html;text/xml;application/xhtml_xml;x-scheme-handler/http;x-scheme-handler/https;
Terminal=false