This improves the implementation for normalizing language tags
when installing a keyboard. Based on the Windows implementation
in `Keyman.System.CanonicalLanguageCodeUtils.pas`.
One difference to the Windows implementation is that the mapping of
3- to 2-letter tags is based on `langtags.json` instead of
`iso639-3.tab`.
Closes#3399.
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.
