feat(QScrollArea): add scroll viewport to create overscrolling effect

[thexeos]

When designing pages (lists, galleries, horizontal scrollers) that have overlays (headers, footers, navigation panels on the sides) using QScrollArea results in either the scroll bar overlapping the content of the overlays (Figure 1) or scroll thumb/bar being hidden (and made inaccessible) by the overlays (Figure 2). In both cases, it leads to confusing experience for users.

If we add offsets to bottom/top of each (vertical and horizontal) scroll tracks, then it will effectively create a scrollable viewport within the container. The scroll behavior remains consistent and predictable to users, while developers get the freedom of creating overlays and adding acrylic/glass effects to their lists. See Figures 3-4.

What kind of change does this PR introduce?

• [X] Feature

Does this PR introduce a breaking change?

• [X] No

The PR fulfills these requirements:

• [X] It's submitted to the dev branch (or v[X] branch)
• [ ] When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• [ ] It's been tested on a Cordova (iOS, Android) app
• [ ] It's been tested on an Electron app
• [ ] Any necessary documentation has been added or updated in the docs or explained in the PR's description.

If adding a new feature, the PR's description includes:

• [X] A convincing reason for adding this feature.

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Other information:

Note 1: there is no documentation in PR yet, until the feature is approved and parameter names are "locked in".

Note 2: this includes code from #17041 and #17207 - once those are merged I will rebased this against the (updated) dev branch.