Added Ability to Pin Chats

[akwotom]

This pull request answers to the demand raised by a user on issue #30.

The following changes were made to achieve this goal:

• A field was introduced to the Threads table, called isPinned (type boolean).
• The ThreadConversationCard was modified to render a new field (isPinned), by showing an icon next to the chat.
• The topAppBar of the ThreadsConversationMain component was modified, introducing two icons, one for pin, and the other of unpin. Clicking each would modify the value of isPinned.

NOTE: This pull request was made from the last used version of lib_smsmms_android, in Deku-SMS 73432ef.