Support "mandatory" constraint for "StringCombo" control

[sanak]

From the following issue's comment, supporting mandatory (=disallow empty) constraint for StringCombo control will be preferable.

• Empty project is possible on form note gtt-project/redmine_gtt_smash#47 (comment)

[moovida]

Ah, I see, yes. Also the whole mandatory thing needs some revision.

At the moment it is possible to safe the note even if mandatory field are not filled.
Since it is possible to complete the form later in the office, I have never placed that block.
What do you think about that?

[sanak]

@moovida (CC: @dkastl, @smellman)
Oh, yes. Just same level constraint as note (string) will be enough, I think.

• Show ( mandatory ) at right side of the label.
• If nothing is selected, show red frame to remind to operator.