Add a background image to the editor

[nils-a]

Only when editing Cake files.
Cake for VS does this:

A starting point could be https://intellij-support.jetbrains.com/hc/en-us/community/posts/115000225424-Editor-opacity-programmatically-

[nils-a]

After discussing this on Slack:

most closest things I could think of is rendering a top-level component of Editor that allows interaction/clicking away

com.intellij.ui.EditorNotifications.Provider