Moved send gifts and inbox to MainActivity

The send gift icon appears next to leaderboards. The inbox button is
below help. I also removed the inventory layout from
SelectModeActivity. It used to contain the inventory, send gift, and
inbox.

Closes #100 Move send gifts to MainActivity
Updated release apk

Incremented the version number from 0.6.5 → 0.6.6 and generated a new
signed apk.