[Bug] Include icons for all sizes in AppImage bundle

[daenney]

What did I do

I tried to package the AppImage for AUR.

What happened

The icon for MQTTX is missing once the resulting desktop file is installed, causing the generic icon to be shown.

Expected

The proper icon is shown.

Environment

• OS: Linux (5.11.6)
• MQTTX version: 1.5.2

More detail

The AppImage archive only includes an icon in the 0x0 directory:

squashfs-root/usr/share
squashfs-root/usr/share/icons
squashfs-root/usr/share/icons/hicolor
squashfs-root/usr/share/icons/hicolor/0x0
squashfs-root/usr/share/icons/hicolor/0x0/apps
squashfs-root/usr/share/icons/hicolor/0x0/apps/mqttx.png

This is problematic, as 0x0 icons aren't picked up on by the system. The icon itself however is actually 512x512, and by moving it into hicolor/512x512/apps instead everything works fine.

This is an Electron build/package issue most likely: https://www.electron.build/icons#linux. It might require upgrading electron-builder.