added replacement class for intl MessageFormatter
as we have a replacement, do not check for intl MessageFormatter
Renamed InstallerPlugin to Plugin
cleaned up Installer.
removed InstallHandler.
adjusted basic app to use the new yii extension installer.
removed Yii::importNamespaces() in favor of the new yii extension installer.
adjusted the advanced app to use the new yii composer extension installer.
Removed re-declaration of $preload from Application
implemented plural and select for fallback intl message formatter
fix tests when intl is not installed
Merge pull request #1044 from cebe/intl-message-fallback

Intl message fallback
make Yii::t fallback behavior consistent with I18N::translate
removed value from client validation message

this is not what is actually beeing validated
Allow ITU message format in validator messages

fixes #991
fixes #991 for gridview summary, use ITU message format
doc wording

[ci skip]
typo ITU -> ICU

they are not responsible for i18n ;)
fixed failing validator tests
fixes #1001