Silence runtime warnings about missing methods

The recent change to set the proper mainwindow on the ktexteditor
views triggered runtime warnings:

warning: kdevelop/unknown: QMetaObject::invokeMethod: No such method KTextEditorIntegration::MainWindow::createViewBar(KTextEditor::View *) [unknown:0/pid=29866]
warning: kdevelop/unknown: QMetaObject::invokeMethod: No such method KTextEditorIntegration::MainWindow::deleteViewBar(KTextEditor::View *) [unknown:0/pid=29866]

This patch adds default implementations for these two functions to
silence the warnings. No behavior is changed otherwise.