Dropdown list support of code auto-completion

[YiqiXu]

Please support code auto-completion with dropdown list as follows

Here are related discussions:

• Is path autocompletion possible?
• Auto completion using popup
• Suggestions by @fbob in issue Modernize UI #4

[YiqiXu]

In the forum, the Label list widget and balloon provides a partial solution.

[da-liii]

@JimZhouZZY will work on it