Updating App Icons

New version of the App Icon: Instead of the black Kiwix bird on transparent (or white)
background, this new version is a lightly-rounded square with a white to grey gradient
that serves as a background for a black Kiwix bird on top.
The main goal of this change is for the icon to be clearly visible on any background;
given OS now support dark/light and custom modes.
I looks quite similar to the iOSmacOS and the Android icons.

In addition, those are now properly resized to all the existing sizes plus a new 512 size.

A new custom Icon for ZIM files is also included and declared for all sizes.
This seems not to be in use or broken but at least the new file is there.
It's a (macOS style) blank document (corner-folded paper) with the black bird on top
with the ZIM label at the bottom

kiwix-desktop.pro

@@ -191,13 +191,21 @@ unix {
   app_icon128.files = resources/icons/kiwix/128/kiwix-desktop.png
   app_icon256.path = $$PREFIX/share/icons/hicolor/256x256/apps
   app_icon256.files = resources/icons/kiwix/256/kiwix-desktop.png
+  app_icon512.path = $$PREFIX/share/icons/hicolor/512x512/apps
+  app_icon512.files = resources/icons/kiwix/512/kiwix-desktop.png
   app_svg.path = $$PREFIX/share/icons/hicolor/scalable/apps
   app_svg.files = resources/icons/kiwix/scalable/kiwix-desktop.svg
-  INSTALLS += app_icon24 app_icon32 app_icon48 app_icon128 app_icon256 app_svg
+  INSTALLS += app_icon24 app_icon32 app_icon48 app_icon128 app_icon256 app_icon512 app_svg
 
   mime_icon48.path = $$PREFIX/share/icons/hicolor/48x48/mimetypes
   mime_icon48.files = resources/icons/48/kiwix/org.kiwix.desktop.x-zim.png
-  INSTALLS += mime_icon48
+  mime_icon128.path = $$PREFIX/share/icons/hicolor/128x128/mimetypes
+  mime_icon128.files = resources/icons/128/kiwix/org.kiwix.desktop.x-zim.png
+  mime_icon256.path = $$PREFIX/share/icons/hicolor/256x256/mimetypes
+  mime_icon256.files = resources/icons/256/kiwix/org.kiwix.desktop.x-zim.png
+  mime_icon512.path = $$PREFIX/share/icons/hicolor/512x512/mimetypes
+  mime_icon512.files = resources/icons/512/kiwix/org.kiwix.desktop.x-zim.png
+  INSTALLS += mime_icon48 mime_icon128 mime_icon256 mime_icon512
 
   mime_file.path = $$PREFIX/share/mime/packages/
   mime_file.files = resources/org.kiwix.desktop-mime.xml

resources/icons/kiwix-app-icons-square.svg

@@ -1 +1,75 @@
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+   viewBox="0 0 512 512"
+   xmlns="http://www.w3.org/2000/svg"
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