Include glib.h instead of glib/$something.h

On some platforms, including a specific file from glib yields a compiler
error, stating that only <glib.h> should be included directly
Use QLineEdit::setPlaceholderText only if Qt 4.7 or newer is used
Use QDateTime::currentDateTime().toUTC() instead of currentDateTimeUtc()

The latter was introduced in Qt 4.7. The former is equivalent and
works with older versions of Qt.