feat(linux): allow installation of arbitrary language tag for package

[mcdurdin]

Keyman for Windows allows the user to specify an arbitrary language tag for a package (in Keyman Configuration post-installation, or via API), but Keyman for Linux will fallback to the default language for the keyboard if it cannot find a matching canonical language tag.

It appears from the command line that you can do, for example:

km-package-install -p sil_ethiopic -l mul

But that doesn't work -- it just installs under am (Amharic). Also, I don't think it is sufficient to return the original language id if no matching better canonical tag is found in:

keyman/linux/keyman-config/keyman_config/install_kmp.py

Lines 225 to 237 in f7989be

	def _normalize_language(self, supportedLanguages, language):
	if len(supportedLanguages) <= 0:
	return ''
	if not language:
	return language
	language = CanonicalLanguageCodeUtils.findBestTag(language, False, True)
	for supportedLanguage in supportedLanguages:
	id = CanonicalLanguageCodeUtils.findBestTag(supportedLanguage['id'], False, True)
	if id == language:
	return id
	return None

Because the ibus registration still only happens for the languages defined in the package. So we'd also need to be registering the 'new' language tag with ibus.

Fixing this may help with #7548