BUG 10920 "Add subtitles inside a drop-down" SOLVED

- New feature, add subtitles inside a drop-down
- Solved problem, added grouping elements in DropDowns
- If a DropDown has enabled optgroup, an example with SQL would be the following:
    (SELECT 'optgroup1' AS UID, 'Users' AS NAME)
    UNION
    (SELECT USR.USR_UID AS UID, USR.USR_USERNAME AS NAME
     FROM   USERS AS USR
     WHERE  USR.USR_USERNAME <> '' AND USR.USR_USERNAME LIKE 'u%'
    )
    UNION
    (SELECT 'optgroup2' AS UID, 'Countries' AS NAME)
    UNION
    (SELECT COUNTRY.IC_UID AS UID, COUNTRY.IC_NAME AS NAME
     FROM   ISO_COUNTRY AS COUNTRY
     WHERE  COUNTRY.IC_NAME LIKE 'b%'
    )
* Available from version ProcessMaker-2.0.47 (2.5.1)