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'
Forms/List: Smarter "ensureVisible(i)" method, to make sure that the
selected list item is near the center rather than at the very bottom of
the list
box.

ensureVisible(i) now jumps to center the item i when i is more than
about half of the list length outside of the currently visible screen.
In all other cases the behaviour is as before.
New Device: Identify Onyx Boox T68 device and set color and epaper type
flags correctly. Does not yet include dither switch problem workaround
(will be separate patch).
OpenGL/Canvas: Implemented InvertRectangle() methods for OpenGL.

Uses GL_BLEND for the inversion, as glLogicOp(GL_XOR) is not reliable on
all implementations.