qt4_editor formlayout now works with colour tuples (fixes Issue #1690)

The form layout wasn't recognising colours that were in 3-tuple form. I added in a function to test for proper 3-tuples (ie: floats between 0 and 1). This is used when setting up the edit form in `FormWidget`'s `setup()` and when compiling the forms value in `get()`.

This fixes issue #1690