Difficulty adjustments & some bug fixes

Lot of small adjustments to improve the experience!

FEATURES

• To cater for those who are unaccustomed to gaming, and those who prefer a gentle introduction to the game, I have introduced a very easy difficulty setting.  This includes more starting lives and food, as well as a slightly longer delay before starting and longer time between dogs.  Powerups last a bit longer.  Speed is halved throughout.  All visual assistance (icons above dogs & aim line) are on.
• Difficulty level is now displayed on-screen and is saved along with the highscore
• The difficulty level now affects the 'star' powerup duration; longer duration at easier levels
• The difficulty level now affects dog speed setting.  A certain number of dogs must be fed before half speed increases to normal speed, and the speed will increase to double speed when a certain number are fed.  At harder levels, fewer dogs must be fed before the speed increases occur.

BUGFIXES

• Tooltips on buttons on the title screen did not always disappear when the game was started - corrected
• On the timer, seconds below 10 displayed as a single digit - now always displays 2 digits
• Depending on screen size, the UI sometimes was cut off - corrected