CheckBoxStyle with padding between image and label

**Problem**
Whenever (read always) I wanted to pad the area between images and text in checkboxes I had to call checkbox.getLabelCell().padLeft(someValue); for all checkboxes. This was a bit cumbersome as I had to remember to call this method for all checkboxes.

**Possible solution**
I've created a solution which adds a new optional padding variable to the CheckBoxStyle. This variable determines the padding between the image and the label.

**Possible future enhancement**
Create a method setPadBetween(float) or padBetween(float) to override the style's padding for single checkboxes.

**Testing**
I've tested this implementation and I'm currently using it in my game :)