Code review comment for lp:~j-corwin/openlp/present

The pptviewlib (which creates preview images on behalf of the client, doing lots of off-screen window grabbing stuff) checks the date of the ppt and compares it to the date of the preview images, and if the ppt is newer then it recreates the images. I was thinking of doing something similar for the others but also create a manual re-load option too.

They can't easily be created when first added to the media manager at the moment, since at this point they don't know which of the three apps is going to display it (or is capable of opening it), and it is these controllers that create the thumbnails. (Also currently each controller has it's own thumbnails).

In the future we might need to specify which controller a file is going to be opened with when it is first loaded, since if you add both a odp and ppt to a service file, the operator isn't going to want to have to remember to change the default controller between each one.