FEATURE: Add signal alternatives to callbacks.

[LennyPhoenix]

Add signals to be used as alternatives for the object & method args in functions.

Related discussion: #3

Progress

• [x] Core
  • [x] Set Log Hook
• [x] Image Manager
  • [x] Fetch
• [x] User Manager
  • [x] Get Current User
  • [x] Get User
  • [x] Get Current User Premium Type
• [x] Activity Manager
  • [x] Update Activity
  • [x] Clear Activity
  • [x] Send Request Reply
  • [x] Send Invite
  • [x] Accept Invite
• [x] New Callback Methods
  • [x] Get Data Callback
  • [x] Get Dimensions Callback
  • [x] Current User Has Flag Callback
• [x] Callback Docs