Product name and directory customization

Mullvad Browser for macOS will not have a portable mode.

So, we need to customize the profile directory not to clash with any other Firefox installation.

Mozilla has a pair of possibilities:

1. explicitly specify Profile= in application.ini (not directly, but through MOZ_APP_PROFILE, which you should set with mk_add_options in the mozconfig)
2. setting the application vendor and name (MOZ_APP_VENDOR and MOZ_APP_BASENAME respectively, the former in browser/confvars.sh, the latter with export in the mozconfig)

The second approach allows to customize the Windows .exe properties, but it has several side effects:

1. it changes the binary name (e.g., for MOZ_APP_BASENAME="TorBrowser" I got torbrowser.exe)
2. it changes the output directory (e.g., obj-whatever/dist/torbrowser)

If we want to follow this approach, we should start with it from the beginning, or updating will be a nightmare (especially the shortcuts).