MainWindow: implement OnMouseDouble()

Opens the menu.  To enable OnMouseDouble(), add the class style
CS_DBLCLKS.
Screen/WList: skip disabled windows in FindAt()

Same behaviour as on GDI.  Required for passing OnMouseDouble() to
parent window.
CrossSectionWidget, GlueGaugeVario: disable user input

Pass user input to the MainWindow.  This allows opening the main menu
with a double click.
BigTrafficWidget: left/right cycles pages

.. instead of selecting traffic items.  On touch screens, we don't
need that gesture, because there are cursor buttons, and on the
Altair, there are other hot keys for moving the cursor.  This fixes a
usability problem (TRAC #2620).
BigTrafficWidget: add "dragging" flag, capture mouse

Don't pass mouse events to GestureManager if not dragging.
BigThermalAssistantWindow: implement gestures

Similar to TRAC #2620.
InputEvents: remove unused function LeaveMode()
InputEvents: make "Pan" a virtual mode

getModeID() returns MODE_PAN if IsPanning() returns true.  Never
actually set current_mode=MODE_PAN.  This allows opening the menu
while panning and fixes a usability bug when opening the flarm radar
or thermal assistant while panning.
Dialogs: add a few "override" and "final" keywords
Makefile: explicitly generate gray-scale PNG files

Works around an ImageMagick bug when implicitly converting to 4-bit
gray-scale PNG.  Fixes dithering artifacts on the vario background
image.
SuspensibleThread: add assertions on IsDefined()

Gets triggered by TRAC #2621.
Startup: update current page after starting the MergeThread

Fixes deadlock when the FLARM page gets initialised by
Pages::Update(): MergeThread::Suspend() deadlocks because the thread
hasn't been started yet (TRAC #2621, #2596).
dlgBasicSettings.cpp + PagesConfigPanel.cpp: Some ascii art we do not need to translate.
Manual: Remaining wrong Altitude term.
dlgConfiguration.cpp: Avoid "modes", instead use "InfoBox Sets".
po: update French translation
release v6.5_beta2