UI: Replace generic parameters with ScrollableWidget

[mike-ward]

Many of the examples in examples/ (e.g. demo_scrollview.v) will immediately fault when compiled with the -prod flag. I've also seen similar faults in my own project, github/mike-ward/Kite. I found that the faults consistently occurred in functions that used generic parameters in Interface_Scrollable.v.

Using a debugger and some printf statements, I determined that pointers in these methods were sometimes corrupt, although I could never deduce why. The corruptions only occurred when building with the -prod flag.

I noticed that the functions in question could have their generic parameters replaced with ScrollableWidget. After making this change, most of the example programs no longer faulted when built with the -prod flag. A few examples, notably, examples\layout\dynamic_layout.v will fault when interacted with. I plan to investigate this later.

I also took the opportunity to remove some redundant checks for has_scrollview() in said functions.

While this issue in not definitive, there is cause to think an issue with generic parameters and -prod may be lurking in V.

[spytheman]

Excellent work.
More concrete code is easier to maintain and debug too,
and it seems that the genericity was not used.