Timezone selection screen

This commit introduces the following changes:
* everything related to timezones is in a separate timezones.py module,
* the timezone-coordinate-to-pixel-function has been severely simplified (watch and learn),
* hence some cc.png colors may have "moved" or had been inconsistent already before — this was also fixed,
* fixed the bug when timezone map-click selection doesn't work correctly on installer window being full screen or resized,
* the timezone screen now shows a timezone-dependent current local time the user can confirm being equal to what they expect,
* the timezone selection combobox has been replaced with a multi-level menu which is a bit easier to preview and less to scroll,
* the radio-style dot.png has been replaced with a red cross,
* the elevation map on the timezone screen has been replaced with a satellite image,
* added was a superposed daylight overlay showing daytime and nighttime places in the world.

Let me know what you think.