Add TStackPanel support class

Inno Setup uses the VCL component library, which includes the TStackPanel component.

Without TStackPanel you would manually set Left, Top and Width on every control, and update all of them in code whenever something changes. TStackPanel automatically lines up its children in a row or column, and reflows them when controls are added, removed or hidden. It saves a lot of tedious positioning code.

TStackPanel is not available in Inno Setup.