BrokenDateTime: add methods IsDatePlausible(), IsTimePlausible()
GlideComputer: check date_available before adding to BrokenDateTime

The operator+ of class BrokenDateTime requires both date and time to
be valid.  For applying the UTC offset, we must consider both cases:
date available (may flip date), or date not available (don't touch
date, stays "invalid").
Terrain/RasterRenderer: fix new[] / delete mismatch
Screen/Color,Bitmap: renaming thecolor and Bitmap::Load

This patch renames thecolor to _color in TopographyFile constructor, just
for naming consistency.

It also renames Bitmap::Load to Bitmap::LoadResource to clarify what it
does, and to distinguish it in the (near) future from a Bitmap::LoadFile
function.

This patch doesn't change anything executable.
doc/manual: require clang 3.2

clang 3.1 may still work, but is not being tested anymore.
DeviceBlackboard: use std::fill() in constructor
DeviceBlackboard: include cleanup
DerivedInfo: require IsPlausible() checks before using date_time_local

There was no formal requirement to check the validity of
date_time_local.  Most users checked NMEAInfo::time_available or
NMEAInfo::date_available, but NMEAInfo is not guaranteed to be in sync
with DerivedInfo.
NMEA/Info: eliminate date_available

Use date_time_utc.IsDatePlausible() instead.
GlideComputerBlackboard: remove unused method time_delta()
po: update xcsoar.pot
Manual: Kick of for a polish translation.