Added pause functionality to Blockrain activity

[VishnuVardhanBR]

Fixes #1499

Any feedback? @llaske

[llaske]

Few remarks:

• At start, the button should be Play
  

• Use the play/pause icon from Fraction activity

• The play/pause button should be before the change theme button

[VishnuVardhanBR]

Pushed a commit according to the mentioned changes. Also made a change so that the user can start the game from the play button in toolbar or in the main screen, is this fine @llaske ?

[llaske]

• It should not be possible to use move/flip buttons when the game is paused
• The help tutorial should be updated to mention the new button

[VishnuVardhanBR]

Made changes, could you please check @llaske ?

[llaske]

• At end of the game the button should become Play and allow to replay the game

[VishnuVardhanBR]

Pushed a commit, could you please check @llaske ?

[llaske]

Nice. Thanks.