Move use gradient colors to Colors tab

The checkbox to switch between coloring schemes (continuous or
discrete), used to be in the 'View' tab but is now found under the
'Colors' tab. The test module code was updated to reflect this changes.

Add a note to the ChangeLog.md file about this change.

Fixes #204
Merge pull request #209 from ElDeveloper/issue_204

Move "Use gradient colors" to Colors tab