Made design changes, added new features.

[RajashekarRaju]

OnBoarding Screen

• Added slider for player to change game difficulty between easy(default), medium, hard.
• Added it's own ViewModel, to update the players game difficulty preferences, show the highest score & level.
• When user taps exit button, preferences for game difficulty will be cleared.

History Screen

• Completed designed history screen, added temporary fake game history details to database. Have few more design changes to make before finalizing this screen.
• Once game screen is stable and able to save the game data, history screen will show real game data.
• Made changes to typography, have to add new typography which isn't creepy.

Game Screen

• Added modal bottom sheet to prevent user from navigating away from current game. Once user taps back in the middle of the game.
• Now game screen shows guessing words based on player selected game difficulty in onboarding screen, if not selected latest value from preferences will be loaded.
• Added few more text elements temporarily to keep track of game changes while playing.
• Need to redesign the whole screen once everything's stable.

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• Added preferences dependencies.
• Added fake game data, fake game history entries.
• Added new column in history table for showing current game level.
• Made use of enums to keep track of game difficulty chosen by the player.
• Completed few tasks added to issue Features & tasks #2