Remove usage of JRouter from Languagefilter Plugin constructor

[Hackwar]

In light of the issues that Falang had recently, this PR removes the usage of the router from the constructor, thus giving plugins like Falangs the chance to do their thing.

[mbabker]

In truth, I think we need to remove the dependency on the language filter plugin from JApplicationSite::initialiseApp(). This constructor is triggered long before the rest of the system plugins are imported for the onAfterInitialise event and I'm not seeing anything in the plugin that makes this dependency a requirement.

[Hackwar]

@mbabker not anymore. This was made possible by #5140. I didn't know that there was this special code in JApplicationSite to do this. But that wont change the issue, just make it harder to detect. So this PR is a good step forward.

[infograf768]

Notice: Use of undefined constant JROUTER_MODE_SEF - assumed 'JROUTER_MODE_SEF' in /plugins/system/languagefilter/languagefilter.php on line 58

[Hackwar]

Fixed.

[gwsdesk]

@test Tested on a site with Falang and wrorks4me. Thanks Hannes