More customizable backgrounds + highscore system

- inside color picker dialog is now possible to select what paint
algorithm (solid colors, gradients, “rainbows”) use to render Petris’s
background
- every score greater than 0 will now be visible in Leaderboards menu
(still working on saving profiles with highscores, preferences and
other stats)
- when the total height of all entries in a menu becomes larger than
the available space, that menu becomes scrollable