GUI: Account for the scroll bar size in ScrollContainerWidget objects

[ccawley2011]

This currently works for ScrollContainerWidgets created using XML layouts, but not ones created using TabWidget::addTab() or OptionsContainerWidget.

Before:

After:

[sev-]

So far, looks proper to me

[sev-]

Merging this because it is already an improvement

[tag2015]

After this PR merge, scrollbars don't show up anymore in the Keymaps and Achievements tabs (on my windows build). Can anyone confirm?

[ccawley2011]

After this PR merge, scrollbars don't show up anymore in the Keymaps and Achievements tabs (on my windows build). Can anyone confirm?

I haven't checked this since the PR was merged, but that was the main reason why I'd originally left this as a draft. I was hoping that someone else would be able to figure out how to get that working, but if this isn't possible in the near future it might be best to revert these changes for now. Sorry for not being more clear about the state of this PR in the initial post.

[schmurtzm]

I confirm : no more scrollbar on x86 Windows or on ARM linux.
Really annoying on handhelds which doesn't have mouse wheel to scroll 😞

[mikrosk]

Slightly (?) related to https://bugs.scummvm.org/ticket/14458, posting it in hope that you could look at that as well. :)