CSVLine: Added IsEmpty() method
Driver: Added basic OpenVario driver
OpenVario: Added static pressure support
OpenVario: Added dynamic pressure support
OpenVario: Added total pressure support
OpenVario: Added temperature support
OpenVario: Added airspeed support
OpenVario: Added TE vario support
Device driver for Wharington Smith Instruments Vaulter
Vaulter: add more documentation, wind support
Vaulter driver: more documentation comments
Vaulter driver: unit test
OrderedTask: allow calls with empty task

Fixes potential crash bugs.
MainWindow: fix crash when switching pages with bottom_widget

The bottom_widget was only visible when there was no "main" widget;
with a main widget, we must not call bottom_widget->Hide(), and
bottom_widget->Move().  This patch adds a few missing
"widget==nullptr" checks.  Fixes TRAC #3012, #3231, #3395.
Makefile: support signed tags in RELEASE_COMMIT_ID formula
release v6.7.6
Merge tag 'v6.7.6'