Refactor scrollbar into more reusable scrollview component

[krissetto]

This PR does a few things:

• Introduces a scrollview component to be used with any components that needs scrolling and a scrollbar;
• Reduces code duplication because of that and simplifies how components interact with a scrollbar, moving most of the scroll specific logic out of components that don't have special edge cases to handle (messages being one of them for now);
• Adapts most scrollable components to use this, for more consistent styling and behavior (mouse support, common keys to scroll with, etc);
• Fixes some minor scrollbar positioning and UX bugs in certain scenarios when dragging the scrollbar with a mouse;

[github-actions]

Code Review Summary

Found 2 issues in the scrollview refactoring:

1. High severity: Double EnsureLineVisible call can hide part of selected messages
2. Medium severity: Mouse hit-testing has 1-frame lag after position changes in dialogs

See inline comments for details.