Add new splash screen and remove unused banner files - fixes #1716 (#1803)

* add new splash screen as decided in #1716

* tweak files to avoid edge artifact

* remove unused banner files

Co-authored-by: Anna Petrasova <kratochanna@gmail.com>

gui/images/startup_banner.txt

@@ -1,35 +0,0 @@
-Startup banner
-
-May 22, 2016
-
-- Updated to capital "GRASS GIS" spelling
-- Contributor:
-  Vincent Bain <bain@toraval.fr>
-
-
-
-Feb 6, 2015
-
-- Startup banner source file corresponding to the latest version uploaded
-  in February : https://trac.osgeo.org/grass/changeset/64480
-
-- The SVG file was generated with Inkscape. Needs the following fonts to
-  display correctly which can be downloaded at:
-
-- Fira sans : http://www.fontsquirrel.com/fonts/fira-sans (common RPM package name: mozilla-fira-sans-fonts)
-- EB Garamond : http://www.fontsquirrel.com/fonts/eb-garamond)
-
-Contributor:
-Vincent Bain <bain@toraval.fr>
-
-------------------------
-Font installation:
-- Fira sans: use package manager
-- EB Garamond: 
-
-sudo su
-cd /usr/share/fonts/
-mkdir eb-garamond
-cd eb-garamond/
-unzip /tmp/eb-garamond.zip
-