Support Quentier app processes discovery and communication about the currently used accounts

[d1vanov]

When a new Quentier app instance is started, it should be possible to discover whether other app instances are running and if so, which accounts (and in which storage folders) they are serving so that two instances won't attempt to manage the same account within the same storage folder.

[d1vanov]

I hoped to use Qt Dbus for this but it appears it is only adequately supported on Linux. Probably it makes sense to implement Dbus interface for Quentier anyway (as Linux is the primary target platform) but some solution for Windows and OS X would be required as well. Need to see whether something useful can be found in boost::interprocess library.

[d1vanov]

Should also look at this - probably not immediately useful but can use the ideas behind the design to implement some shared memory-based solution.