ENH: Improve about dialog appearance

Tweak about dialog to use the new logo at 100% size, as intended. Remove
background from text portions. Remove forced color palette. This
significantly improves the appearance and consistency with e.g. the
Slicer Dark style or the (KDE) Oxygen style, especially with other than
the brutally-white color palette.
ENH: New Slicer logo, take 2

Recreate the slicer logo (again) in a smaller size that more closely
matches the original default size of the About dialog. (The old dialog
was using a downsampled version of the original image!) This one has the
major benefit of also being an appropriate size for the splash screen,
which will enable us to recreate that as well.

To facilitate that, this version separates the logo from the text, and
combines the components via a Gimp .xcf file.

The (new) new version is essentially the same as the previous version,
tweaked for pixel alignment in the new size. The text has also been
hand-kerned for aesthetics and pixel alignment.
ENH: New version of Slicer splash screen

Create an updated version of the Slicer splash screen using the new
version of the logo, and with heavy hinting applied to the 'supported
by' text to improve visual "crispness".
ENH: More tweaks to new logos

Adjust the XY and YZ plane grid elements to have less Y extent. This
more closely matches the proportion of the old logo (and also looks
better; already these are significantly elongated in the Y dimension
compared to true squares, so lessening that is an improvement), at the
trade-off of more 'empty' space along the Y axis versus the old logo.
(However the old logo also severely distorted the border between outer
and inner rings along the Y axis, which made the inner 'core' smaller
along the Y axis to begin with.)