Some point before commit #bfa28f1 in the frameworks branch a change was introduced that leads to a loader failure when starting Okular on OS X.

This turns out to be caused by the fact that libokularplugin.dylib is linked with okularpart.dylib, and okularpart.dylib has an incorrect identity stored in it:

```
> otool -L /opt/local/share/qt5/qml/org/kde/okular/libokularplugin.dylib
/opt/local/share/qt5/qml/org/kde/okular/libokularplugin.dylib:
        /opt/local/lib/libokularplugin.dylib (compatibility version 0.0.0, current version 0.0.0)
        /opt/local/libexec/qt5/Library/Frameworks/QtQuick.framework/Versions/5/QtQuick (compatibility version 5.6.0, current version 5.6.1)
        /opt/local/libexec/qt5/Library/Frameworks/QtQml.framework/Versions/5/QtQml (compatibility version 5.6.0, current version 5.6.1)
        /opt/local/lib/okularpart.dylib (compatibility version 0.0.0, current version 0.0.0)
        /opt/local/lib/libOkular5Core.7.dylib (compatibility version 7.0.0, current version 7.0.0)
[SNIP]
```

Note the expected install path for okularpart.dylib, which is not correct but stems from:

```
> otool -L ldd /opt/local/share/qt5/plugins/okularpart.dylib
/opt/local/share/qt5/plugins/okularpart.dylib:
        /opt/local/lib/okularpart.dylib (compatibility version 0.0.0, current version 0.0.0)
        /opt/local/lib/libOkular5Core.7.dylib (compatibility version 7.0.0, current version 7.0.0)
        /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1197.1.1)
        /opt/local/lib/libphonon4qt5.4.dylib (compatibility version 4.0.0, current version 4.9.0)
[SNIP]
```

When I build Okular with the same configuration and install target settings on Linux there is no such error. In fact, libokularplugin.so does *not* reference okularpart.so, meaning that the linker editor determined that nothing from that library is used except for its public link libraries.

A basic workaround is to create a symlinked /opt/local/lib/okularpart.dylib that points to the binary in Qt5's plugin directory. 
The most correct fix would be to correct the identity stored in that file on OS X, but this is somewhat debatable given that the library isn't required (and I'm not sure how to do this in cmake).

Another fix which takes that particular fact into account is to adjust the target_link_libraries for the okular plugin, in mobile/components/CMakeLists.txt :

```
target_link_libraries(okularplugin
        Qt5::Quick
        Qt5::Qml
        Qt5::Core
        Qt5::Xml
        Qt5::Svg
        okularcore
        KF5::Bookmarks
        KF5::I18n
        KF5::IconThemes
        )
```

This works on both Linux and OS X. On Linux, the resulting output from `ldd libokularplugin.so` is unchanged, which confirms the idea that okularpart.so is only used to link the KF5 Bookmarks, I18n and IconThemes frameworks indirectly.

Reproducible: Always

Steps to Reproduce:
1. checkout and build the frameworks branch at #bfa28f1
2. install
3. run

Actual Results:  
A dynloader error that okularpart.dylib cannot be found

Expected Results:  
no errors