Beautify MacVim's dmg installer background and volume icon

The existing background image for the MacVim dmg installer was quite
old. It did not have a 2x retina version, and design-wise it showed a
redundant MacVim logo when there's already a draggable MacVim app with
the same icon. Update the background image for a cleaner refreshed
design. Image was designed by Jason Long (@jasonlong).

Also, change the dmg's volume icon. We were using the MacVim logo as the
volume icon, but when mounting the image, it could lead to a confusing
state where the user sees a MacVim logo on the desktop, thinking it's
the app itself, but it's actually a dmg volume. Instead, use a new
designed volume icon with the macOS disk image icon overlaid with Vim's
logo/color for styling to make it clear it's a volume, not an app.

Author: ychin

src/MacVim/Credits.rtf

@@ -1,4 +1,4 @@
-{\rtf1\ansi\ansicpg1252\cocoartf2709
+{\rtf1\ansi\ansicpg1252\cocoartf2821
 \cocoascreenfonts1\cocoatextscaling0\cocoaplatform0{\fonttbl\f0\fswiss\fcharset0 Helvetica-Bold;\f1\fswiss\fcharset0 Helvetica;\f2\fswiss\fcharset0 Helvetica-Oblique;
 }
 {\colortbl;\red255\green255\blue255;}
@@ -51,15 +51,10 @@ Dmitry Vazhov\
 \f1\i0 \cf0 \
 ...and many others who have helped by reporting bugs etc.\
 \
-\
-Thank you to {\field{\*\fldinst{HYPERLINK "http://www.positivespinmedia.com"}}{\fldrslt Positive Spin Media}} for the PSMTabBarControl Framework.\
-\
+\pard\tx566\tx1133\tx1700\tx2267\tx2834\tx3401\tx3968\tx4535\tx5102\tx5669\tx6236\tx6803\pardirnatural\partightenfactor0
+\cf0 The MacVim icon was designed by {\field{\*\fldinst{HYPERLINK "https://jasonlong.me/"}}{\fldrslt Jason Long}}.\
+\pard\tx566\tx1133\tx1700\tx2267\tx2834\tx3401\tx3968\tx4535\tx5102\tx5669\tx6236\tx6803\pardirnatural\partightenfactor0
+\cf0 \
 Thank you to {\field{\*\fldinst{HYPERLINK "https://whomwah.github.io/qlstephen/"}}{\fldrslt Duncan Robertson}} for the QLStephen QuickLook plugin.\
 \
-Toolbar icons by {\field{\*\fldinst{HYPERLINK "http://www.mattballdesign.com/"}}{\fldrslt Matt Ball}} (free Developer Icons), {\field{\*\fldinst{HYPERLINK "http://www.jonasraskdesign.com/"}}{\fldrslt Jonas Rask}} (Danish Royalty Free icon set), and {\field{\*\fldinst{HYPERLINK "http://www.everaldo.com"}}{\fldrslt Everaldo Coelho}} (Crystal Project Icons, released under LGPL license).\
-\
-Revamped MacVim icon by RIC.\
-\
-Thanks to Andy Matuschak for {\field{\*\fldinst{HYPERLINK "http://sparkle-project.org"}}{\fldrslt Sparkle}}.\
-\
-Thanks to Damien Guard for {\field{\*\fldinst{HYPERLINK "http://damieng.com/blog/2008/05/26/envy-code-r-preview-7-coding-font-released"}}{\fldrslt Envy Code R}} (used in MacVim's 16x16 document icons).}
+Thanks to Andy Matuschak and the Sparkle Project for {\field{\*\fldinst{HYPERLINK "http://sparkle-project.org"}}{\fldrslt Sparkle}}.}