Missing icons in AppImage build

[dist3r]

Since version v1.0.217 (probably starting from commit 279310f) AppImage build is not compliance with AppDir specification when it comes to icons directories. Icons (there might be multiple icons with different resolutions) should be placed in directories which follows the rule: <root>/usr/share/icons/<theme>/<resolution>/apps/myapp.<ext> but instead there is only one directory: <root>/usr/share/icons/hicolor/0x0/apps/joplin.png. As there couldn't exists any picture with 0x0 resolution, tools such as AppImageLauncher fails extracting icon and Joplin gets generic picture in application launcher (e.g. in GNOME).

Environment

Joplin version: v1.0.217 - v1.0.242 (newest at the moment)
Platform: Linux
OS specifics: Fedora 32 with GNOME (but probably any other Linux OS)

Steps to reproduce

1. Download affected Joplin version
2. Install app using AppImageLauncher
3. Start app/look for it's entry in desktop app launcher

If you want to see icon's directories structure do the following:

1. Download affected Joplin version
2. Extract files from AppImage file using --appimage-extract switch, e.g. ./Joplin-1.0.217.AppImage --appimage-extract
3. Cd into squashfs-root/
4. Examine previously described directories structure

Describe what you expected to happen

I'm providing the fix for this issue in #3721. Please review it.

Related issue

#3529
#3428