[WIP] Implement open & select file in file manager

Should close #218 when ready for merge.
**[WIP] Not ready for merging. Putting it here for a discussion**

As you can see, implementing this will introduce three platform-specific code branches, which is not very convenient.

**[DONE]** On Windows: using explorer.exe command line switch
On Linux we'll need to use DBUS: http://www.freedesktop.org/wiki/Specifications/file-manager-interface/
On Mac: dunno atm