Remove qt4 moc files and add moc to buildsystem

This is neccessary to conditionally enable signals/slots
at build time.

* qt4/Makefile.am: Moc files automatically.
* qt4/pinentryconfirm.moc, qt4/pinentrydialog.moc,
qsecurelineedit.moc: Removed.

• While this removes the hard version requirement to Qt 4.8.2 caused by the old generated files it breaks the possibillity to compile pinentry-qt and pinentry-qt4 in one go as the MOC variable is only set once either the qt4 variant or the qt3 variant will be broken. At least there is a decent error message in that case.