Add validation support to ComboBox

[Al-Dyachkov]

It would be handy to provide user feedback when combo box value is required, with the appearance similar to TextBox one

[Sicquze]

wen :(

[SKProCH]

When i have free time to redo ComboBox, sadly. But adding a validation only isn't a hard thing, i'll try to find a time in a weekend. If somebody want to submit a PR is would be highly appreciated

[Sicquze]

When i have free time to redo ComboBox, sadly. But adding a validation only isn't a hard thing, i'll try to find a time in a weekend. If somebody want to submit a PR is would be highly appreciated

thx for fdbk