Update NVDA icon

See PR #7446

Include the source `svg` so that future modifications can be made more
easily.

The previous dark grey shaded icon has been in use for nearly 10 years.
It was first created for Windows XP, which had a light grey system tray
which showed up the dark grey logo very well.  Recent versions of
Windows have moved towards a darker system tray.  There is very little
contrast with the existing NVDA icon.  The updated icon features a
similar logo in flat white, on a solid purple background. This ensures it
will be visible on any colour background, and uses the purple from the
NV Access logo.

The NVDA logo is a stylised blend of the letters NVDA. It has a vertical
line on the left with a little hook at the bottom. Then from the top, a
straight diagonal line comes down to the right. Next, a line curves in a
semi-circle out to the right then back up to the top, and finally there
is a little hook down to finish. The vertical and diagonal lines make
the first part of the letter N and also the letter A (without the cross
bar). The diagonal and curved lines form a V, and the curve is the right
part of the D.

user_docs/en/changes.t2t

@@ -59,6 +59,7 @@ Highlights of this release include input of contracted braille, support for new
  - Previously, it always showed as much context information as possible, regardless of whether you have seen the same context information before.
  - You can revert to the old behaviour by changing the new "Focus context presentation" setting in the Braille Settings dialog to "Always fill display".
 - When using Braille, the cursor can be configured to be a different shape when tethered to focus or review. (#7112)
+- The NVDA logo has been updated. The updated NVDA logo is a stylised blend of the letters NVDA in white, on a solid purple background. This ensures it will be visible on any colour background, and uses the purple from the NV Access logo. (#7446)
 
 
 == Bug Fixes ==