GlueMapWindowDisplayMode: Check availability of heading for DisplayMode::HEADING_UP

Fixes trac issue #2662, #2663.
GlueMapWindowDisplayMode: Simplify code

Don't try to handle non-availibility of basic.track for
DisplayMode::TRACK_UP in if branch of DisplayMode::NORTH_UP.
test/harness_task: obtain task factory handle after SetFactory()

Fixes valgrind warnings and false negatives.
MainWindow: disable gestures while modal dialog is shown

Fixes random freezes because the MainWindow was capturing all touch
events (TRAC #2664).
Android release v6.5.1b

Ship the fixes for the two Android-specific freeze bugs quickly to our
users.
Manual de: Move to screen readable font. Three chapters reworked.