Godot 3 do not import PBM files (no .import file) for Click Masks

[nylnook]

Godot version:
Archlinux 64b, Godot 3b1

Issue description:
TextureButtons Click Masks seem to be badly imported in Godot 3, and do not generate .import files in the project folder
This might be the cause of #14828
See also #14700 for a sum-up in and a minimal project example

[reduz]

This is not a bug, and seems to be a duplicate of #14700, so closing

[nylnook]

As explained in #14828, we are speaking about PBM... @akien-mga asked me to separate feature proposal and bugs, which is why it is separated from #14700 ... please reopen !

[nylnook]

thanks @akien-mga ;)

[reduz]

PBM format is now gone, you can import an image as bitmap