Fixed: show placeholder string while editing empty CPTextField

Previously, selecting an empty text field hid the contentView, causing the placeholder string to dissapear. This fix leaves the contentView of the text field visible when the internal _stringValue is nil or empty.

Fixes #2049
Test: added manual test case to check for placeholder.

Refs #2049
Merge pull request #2097 from Asriwi/fix-textfield-placeholder

Fixed: show placeholder string while editing empty CPTextField