Blender Git Loki

Don't use split layout for operator properties if all are checkboxes

Having a split layout looks broken if all items are checkboxes and there
is no heading text, because it only uses half the width then.
So this checks if all visible properties are booleans and if so,
disables the split layout. A bit hacky but this should generally work.
If needed we can add more control in future (e.g. default layout hints
for operators).