This allows to treat empty parameters as default

This new wrapper ensures types are correct and accessed parameters are
actually set (with custom default fallbacks).

The wrapper is available in the global $INPUT variable. It accesses
$_REQUEST by default. If POST or GET is required, the post and get
members can be used:

$INPUT->int('foo',false); // access $_REQUEST['foo'], default false
$INPUT->post->int('foo'); // access $_POST['foo'], default 0
$INPUT->get->int('foo'); // access $_GET['foo'], default 0

The codebase still needs to be updated to make use of this.

Fix for FS#2522 / Now all places of $lang['restored'] are covered with the restored-date information

This also fixes a problem wiht PHP 5.4 when there is metadata but the
date key is empty.

This bug occurs on systems where writing a zero-length string to an
empty file does not update the file's modification timestamp.

This leads to the messages.txt being downloaded almost endlessly, causing
long delays for logged-in users. Visitors are not affected, because the
messages.txt is only updated for logged-in users.

When a plugin file exists, we can assume it is the correct file and load
it without error supression. This makes it much easier to detect and
debug problematic plugins.

In some circumstances the username was set in latin1.

This prevents an "Illegal string offset" error in PHP 5.4 in the test
cases (the integration tests failed here with PHP 5.4).

Previously when in an event handler the same event was triggered again,
only the handlers for the second event invocation were all executed, the
handling of the "outer" event stopped after the handling of the inner
event as they both used the same iterator of the hooks array. This
caused caching bugs e.g. when both the include and the indexmenu plugin
were enabled as both of them load metadata in the cache handler which
triggers another renderer cache event.

This allows easier identifying of which version has been restored. (FS#2522)