Add Transparency to placeholder

[paer76]

is your feature request related to a problem?
no

Describe the solution you'd like
Take into account the alpha property of the resources used for shading the placeholders.

Describe alternatives you've considered
None

Additional context
None

[piiertho]

Currently when you are placing a placeholder on a map it is temporary and have alpha hard coded in editor plugin. Then when placeholder is added editor plugin set alpha back to one. I think the simplest thing we can do is simply multiply the current hard coded alpha values set in editor plugin with placeholder type alpha.