Custom Property Settings - List the showMode property

[JaneSjs]

Please add the following setting to a list of custom property settings: showMode: form.

https://surveyjs.io/survey-creator/documentation/property-grid#survey-element-property-settings

[RomanTsukanov]

If we are going to publish this functionality, we need to change the default behaviour: place properties in the form because rows cannot fit many editors and do not support some editor types properly (dropdown, for example). We also need to change the API to a Boolean property: showInForm: true (default) | false

[JaneSjs]

+1 T17555 - Custom properties on matrix columns https://surveyjs.answerdesk.io/internal/ticket/details/T17555

[JaneSjs]

T19403 - Adding a property to the edit section of choices (itemValue[]) https://surveyjs.answerdesk.io/internal/ticket/details/T19403