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".