IpodCollection: more robust iPhone (and the like) mounting

CHANGES:
* determined mount-point is checked first for some well-known
directories to detect whether iPhone is already connected
* detailed debugging information is shown to the user in details tab
of a sorry dialog upon failure -> no need to run amarok with --debug
* output of the called commands is now shown in the details too to aid
debugging
* the code is significantly refactored to eliminate convoluted logic

This is not the final solution of course - that would be using solid to
do all the dirty job, and thanks to work on fuse backed this may one
day become true.

I'd like to thank avlas for testing as seen on my blog post [1].

[1] http://strohel.blogspot.com/2012/04/amaroks-rewritten-ipod-plugin-testers.html

DIGEST: Bugfix: Amarok's iPhone mounting gets more robust