Option to disable label rectangle background

**Is your feature request related to a problem? Please describe.**
Overlaying a color on top of the selected region can affect the perception of features by which objects are labelled, therefore leading to incorrect labels. This is mostly an issue when using pre-annotations.

**Describe the solution you'd like**
Add an option to disable the background for label rectangles and only keep the colored border.

**Describe alternatives you've considered**
Hiding the label region temporarily, which is tedious.