Improvements to install adapters

[mbabker]

• Restored preflight method to plugin uninstall per @pasamio comments at Handling msg object in adapter uninstall #1343 (comment)
• Added check to package adapter install method to determine whether to use install or update route; subsequently changed later JInstaller::install trigger to use the route information
• Document class vars in template adapter
• Remove check for install root tag in JInstallerHelper::detectType()
• Deprecate JInstallerHelper::splitSql() in favor of JDatabaseDriver::splitSql() (which the deprecated method already proxies to)
• Remove obsolete code per @infograf768 as suggested at Use language pack name as 'element' name instead of language pack #953 (comment)