New icons and artwork

As title says, I prepared some new icons for XBMC but was not yet able to test how they look and behave in real use, so depending on your feedback icons might have to be adjusted.

I digged into the design guidelines of all platforms besides Linux (are there any?) and created icons in according sizes. What's left to do now is to actually use those icons in our buildprocesses and probably also try to get rid of dupes and move all icons to "media" folder.
For now I grouped platform specific icons in their own folders so that devs find them more easily, but we can also change this structure so that icons can more easily be reused among platforms (if sizes match).

For OSX I was not able to build the incs file, but @jmarshallnz said that this could easily be created during build process. I tried to use correct naming (guidlines said that the folder has to be named like that), but feel free to move/rename stuff.

All other platform devs should also please have a look and make use of the new icons (like the hdpi icons provided for iOS and Android). For Windows I have created an .ico file which we could use. Feel free to create PRs/commits to my branch or just ping me what to change code wise and I'll add it to this PR and bring it in shape once everything is in place.