Add ContainerControl to Web projects

Web projects currently offer three containers for nesting controls: GroupBox, PagePanel and TabPanel. Desktop projects additionally have ContainerControl, a reusable container you design once in its own designer (controls, properties and event handlers included) and then place on any window.

Requested change: bring reusable containers to Web projects.

• A new Web Container Control item in Web projects, designed like a Web page with the same responsive Compact, Medium and Wide breakpoint grids.
• Instances are placed on any Web page from the Control Library and carry their controls and event handlers with them, so a shared header, sidebar or form group is maintained in one place.
• The container exists in both the desktop and web standard libraries under the same name, so shared code that walks child-capable controls with Controls(), FindControl and FindControlByTag compiles and runs against both targets.
• As with GroupBox on the web, the layout stays designer-owned: instances are placed at design time, and the runtime mutation methods (Add, Remove, RemoveAll) remain desktop-only.