[users] Use libpwquality for additional password checks

- add cmake module to find libpwquality
 - move checking functions to their own file
 - some Transifex hackery
 - stub out the libpwquality check

src/modules/users/UsersPage.h

@@ -26,9 +26,9 @@
 
 #include "Typedefs.h"
 
-#include <QWidget>
+#include "CheckPWQuality.h"
 
-#include <functional>
+#include <QWidget>
 
 namespace Ui
 {
@@ -70,41 +70,7 @@ signals:
 private:
     Ui::Page_UserSetup* ui;
 
-    /**
-     * Support for (dynamic) checks on the password's validity.
-     * This can be used to implement password requirements like
-     * "at least 6 characters". Function addPasswordCheck()
-     * instantiates these and adds them to the list of checks.
-     */
-    class PasswordCheck
-    {
-    public:
-        /** Return true if the string is acceptable. */
-        using AcceptFunc = std::function<bool( const QString& )>;
-        using MessageFunc = std::function<QString()>;
-
-        /** Generate a @p message if @p filter returns true */
-        PasswordCheck( MessageFunc message, AcceptFunc filter );
-        /** Yields @p message if @p filter returns true */
-        PasswordCheck( const QString& message, AcceptFunc filter );
-        /** Null check, always returns empty */
-        PasswordCheck();
-
-        /** Applies this check to the given password string @p s
-         *  and returns an empty string if the password is ok
-         *  according to this filter. Returns a message describing
-         *  what is wrong if not.
-         */
-        QString filter( const QString& s ) const
-        {
-            return m_accept( s ) ? QString() : m_message();
-        }
-
-    private:
-        MessageFunc m_message;
-        AcceptFunc m_accept;
-    } ;
-    QVector<PasswordCheck> m_passwordChecks;
+    PasswordCheckList m_passwordChecks;
 
     const QRegExp USERNAME_RX = QRegExp( "^[a-z_][a-z0-9_-]*[$]?$" );
     const QRegExp HOSTNAME_RX = QRegExp( "^[a-zA-Z0-9][-a-zA-Z0-9_]*$" );