Improvement suggestions

Hi Daniel

This PR covers 3 areas:
* Fix for the uninstall/reinstall issue: Renaming /assets/install.xml to /assets/install.xmt so Joomla doesn't threat it as valid manifets file anymore on uninstall. Also changing where the file gets loaded and checked.
* Renaming the constructed manifest file for the languagepack from install.xml to "<extension>-<language>.xml". This makes sure when installing multiple languagepacks the Joomla extension manager doesnt' just overwrite the first installed with the second.
* Adding the language code to the <name> tag in the manifest file so the user is able to distinguish multiple language packs in the extension manager.