[Tasks] Move "Add a task"

[jo-spek]

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