Refactor with fragments

Use fragments for each of message / result grid / image detail screens, instead of the previous custom view
switching with backstack-like functinality through the event bus.
This allows to streamline the code, separate each screen rendering more clearly and build upon the framework
with state saving through arguments and navigation through the fragment transactions and the backstack.
Update icon

Replace default robot icon with a custom one.
Remove action bar
Move search bar a bit down