When installing a keyboard with a bcp47 tag we now look up the
language tag in the keyboard kmp.json file. This fixes#3399 in
case a script subtag is required for the language and will cause
the keyboard to properly show up in the Linux keyboard selection
list.
This change implements the custom keyman:// protocol handler which
will cause the browser to launch km-config when it encounters a
URL using the keyman protocol.
Related to #3271.