Check if a LibreOffice 5 installation is available and disabled loviewer when it is not

[Stefano Verzegnassi]

Currently we don't handle any exception in the Libreoffice plugin.

We should check that the path specified for lok::lok_cpp_init() exists[1][2], and s_office[1] has been properly initialized.
If not, docviewer should show a Dialog, which warns the user of the error and suggest to install LibreOffice 5 on his desktop.

(On devices we'll provide LibreOffice inside the docviewer package, so no error will be prompted. See [3].)

[1]: http://bazaar.launchpad.net/~verzegnassi-stefano/ubuntu-docviewer-app/reboot-lok-qsg-zoom/view/head:/src/plugin/libreofficetoolkit-qml-plugin/lodocument.cpp
[2]: http://bazaar.launchpad.net/~verzegnassi-stefano/ubuntu-docviewer-app/reboot-lok-qsg-zoom/view/head:/src/plugin/libreofficetoolkit-qml-plugin/config.h
[3]: https://code.launchpad.net/~verzegnassi-stefano/ubuntu-docviewer-app/get-click-deps