Extend spec to give any app icons in usr/share/icons preference

Draft for approaching #19 (as well as #16):
* mention icon files below `usr/share/icons/hicolor` to be taken into account preferred if present
* use $APPICON as variable name for the application icon identifier, to remove the otherwise unneeded expectation for being the same string as used for the basename of the desktop file name (the `Icon=` would be unneeded otherwise)

Author: kossebau

draft.md

@@ -143,7 +143,8 @@ An [AppImage] which conforms to the type 2 image format:
 * **MUST** contain a file named `AppRun` in its root directory
 * **SHOULD** contain a [payload application] that is ultimately executed when the [AppImage] is executed
 * **SHOULD** contain exactly one `$APPNAME.desktop` file in its root directory with `$APPNAME` being the name of the [payload application]
-* **MAY** contain an `$APPNAME.png` file in its root directory with `$APPNAME` being the name of the [payload application] as set in the `Icon=` key of the `$APPNAME.desktop` file. If present, this icon **SHOULD** be given preference as the icon being used to represent the [AppImage]. The icon **SHOULD** be a PNG with 256x256 or 512x512 pixels
+* **MAY** contain icon files below `usr/share/icons/hicolor` following the  [Icon Theme Specification](https://standards.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html) for the icon identifier as set in the `Icon=` key of the `$APPNAME.desktop` file. If present, these icon files **SHOULD** be given preference as the icon being used to represent the [AppImage].
+* **MAY** contain an `$APPICON.svg`, `$APPICON.svgz` or `$APPICON.png` file in its root directory with `$APPICON` being the icon identifier as set in the `Icon=` key of the `$APPNAME.desktop` file. If present and no icon files matching the icon identifier present below `usr/share/icons/hicolor`, this icon **SHOULD** be given preference as the icon being used to represent the [AppImage]. If a PNG file, the icon **SHOULD** be of size 256x256, 512x512, ot 1024x1024 pixels.
 * **SHOULD** contain a `.DirIcon` file as per the [AppDir] specification
 
 #### The `AppRun` file: