UIEnvelope requests too little space

[burksbuilds]

The UIEnvelope class requests a preferred size based on calling the UISize.cumulateBoth() function to add together the content preferred size and the width/height specified in the 'border' UISize field. The cumulateBoth function adds the width and height of the input UISize once to the output UISize. However, the 'width' and 'height' fields of the UISize border field represent a symmetric thickness that should be applied to both sides of the contained widget, so they should actually be added TWICE to the output UISize.

[dirkhillbrecht]

Merged into master, will be included in next release