Theme customizing hooks

Added some hooks required for custom theming:

- Template.addBeforeMenuContent - was used for adding breadcrumbs. Parameter is used to pass what should be last element of breadcrumbs, according to active plugin.
- Template.addExtraContentTopBar - this allows to formulate own top menu, not necessarily split into two twig templates. All variables used in respective templates are passed into this hook, allowing to apply own logic.
- LanguageManager.alterAvailableLanguages - allows plugins to nicely limit available languages to only defined list.
- Template.loginScreenExtraAssets - allows to inject additional assets to modify login screen look
- Template.footerAddExtra - allows to inject some custom footer into Piwik