Apple/InternalSensors: Fix compilation with FIXED=y

Add fixed casts for assignment of double values to variables of type
fixed
Apple/InternalSensors: Add location data validity checks

According to Apple's CLLocation Class Reference, the longitude and
latitude values are invalid, if horizontalAccuracy is negative, the
altitude value is invalid, if verticalAccuracy is negative, and the
course value is invalid if negative.
Apple/InternalSensors: Set gps.hdop value

Set NMEAInfo gps.hdop value to the horizontalAccuracy received from the
CoreLocation API
manual: GPS priority when using multiple sources

see http://forum.xcsoar.org/viewtopic.php?f=3&t=568#p2565
manual-de: device priority when using multiple sources
doc: document build process for iOS and Mac OS X
Merge branch 'apple' of git://git.xcsoar.org/xcsoar/felix/xcsoar
ObservationZoneClient: use nullptr instead of NULL
Util/Manual: add method Get()

May be more clear than operators.
Util/Manual: add assertion to Get()
Util/Manual: use Get() in Destruct()
Util/Manual: cast via void* to avoid alignment warnings
Util/Cache: use the Manual<> template

Replace the local Constructible<> template which was essentially the
same.
OpenGL/CanvasRotateShift: Appropriate scaling when using GLSL

This fixes #3279
Makefile: Create xxhdpi and xxxhdpi Android app icons

Create the App icon in xxhdpi and xxxhdpi resolutions to get sharper
icons on high resolution Android devices
RoutePlanner: eliminate redundant set::find() call

set::insert() returns whether insertion took place.  This eliminates
some runtime overhead by searching the tree only once.
RoutePlanner: don't use std::tr1::_Fnv_hash

This is an undocumented internal API that has disappeared long ago.
RoutePlanner: remove "tr" namespace

Now that C++11 has been released, the "tr1" namespace is obsolete.
RoutePlanner: use std::unordered_set
RouteLink: remove obsolete operator<