New demo - Vue3 render functions with GridStack.renderCB

[abulka]

Description

New demo of Vue3 render functions using GridStack.renderCB

Benefits

• less code than using GridStack.addRemoveCB, since GridStack creates the 2 needed parent divs + classes
• more information passed to GridStack.renderCB() - (entire GridStackWidget is passed so you can use id or some other field as logic)
• less templating needed (no need for div class="grid-stack-item wrapper) in each Vue component
• fixes {{ info }} display

Checklist

• Created tests which fail without the change (if possible)
• All tests passing (yarn test)
• Extended the README / documentation, if necessary

Checklist N/A

[adumesny]

thank you for making this demo! let's make it linked from the index page as well.