Add support for dropdown filter default option text

Add support for dropdown filter default option text, just like column headings.

Use 'dropdown_title' as the from / to label key.
Fallback dropdown title to column title if set

If a dropdown_title label is not found. fallback to column_title, if it is set, before using the default $this->title.
Merge pull request #400 from GaryJones/patch-1

Add support for dropdown filter default option text