katemdi: Fix deprecation warnings

Summary:
```
/home/kfunk/devel/src/kf5/kate/kate/katemdi.cpp:322:5: warning: 'appendTab' is deprecated [-Wdeprecated-declarations]
    appendTab(icon, newId, text);
    ^
/home/kfunk/devel/install/kf5/include/KF5/KWidgetsAddons/kmultitabbar.h:138:35: note: 'appendTab' has been explicitly marked deprecated here
    KWIDGETSADDONS_DEPRECATED int appendTab(const QPixmap &pic, int id = -1, const QString &text = QString());
```                                  ^

The alternative appendTab function is part of KF5 5.13, thus we need a
version bump.

Reviewers: cullmann

Reviewed By: cullmann

Subscribers: cullmann, kwrite-devel

Differential Revision: https://phabricator.kde.org/D6132