[Tasks] Move "Add a task"

**Tl;dr: Move "Add a task" from the bottom to the right like in Google Forms.**

Currently, adding tasks somewhere specific in a bigger survey is quite cumbersome, as a new task is by default added at the bottom as last task, and moving it up requires quite the abarticulation of keeping right-mouse clicked while turning the scroll wheel.

Instead, it would be elegant to have the "Add a task" pane **scroll along with the users scroll**, and **adding a new task places it at the current position** instead of at the end.

Certainly not of high priority, but worth to think about it.

@gino-m @vittorino @rfontanarosa

No files or tests are named. Reproduce the survey task editor and locate the UI entry point for the “Add a task” pane; compare its behavior while scrolling and inserting tasks. Done means the pane follows the user’s scroll position and a new task is inserted at the current position rather than always at the end.

Written by the indexing model from the issue text.