Ensure that AppImages contain valid .DirIcon file

[probonopd]

• Version: Latest
• Target: Linux AppImage

Every AppImage needs to contain a top-level .DirIcon file. Please use a 256x256 px png. (linuxdeployqt puts the main icon specified in the desktop file there, but electron-builder is not using linuxdeployqt.)

References:

• https://github.com/AppImage/AppImageSpec/blob/master/draft.md#the-filesystem-image
• Add rule that .DirIcon should be png AppImage/AppImageSpec#27
• AppImage is missing a .DirIcon file geraldoramos/nikola#31
• AppImage is missing a .DirIcon file wagerr/wagerr-electron-app#4
• AppImage is missing a .DirIcon file neopostmodern/wohnungsbot#1

cc @develar

[develar]

Hmm.... In this line iconFileName should be changed to .DirIcon. I was under impression that electron-builder creates some explicitly named icon file. Have to check it again. Cannot believe :)