Port and update the Search Bar plugin

kDebug(), qDebug() -> qCDebug() with logging category
Use Qt5 signal connection syntax
No need to link with KDELibs4Support
KLocale -> QLocale
QUrl::addQueryItem() -> QUrlQuery
QWeakPointer -> QPointer
QFontMetrics::width() -> horizontalAdvance()
Warning regarding deprecated QScriptEngine

Using a QWeakPointer as a tracked pointer to a QObject is now
deprecated as obsolete, so the most appropriate type to use for
maintaining a reference to the associated KPart would seem to
be a QPointer.