fix(lt-body): scrollToRow before render

[FalkF]

When changing the scrollToRow before newly added rows are rendered targetScrollOffset can not be calculated:

this.table.addRows(data);
const targetRow = this.table.rows.find(element => element.get('name') === 'target');
set(this, 'scrollToRow', targetRow);

This can be worked around by changing scrollToRow in the next runloop, but it would be cool to just make it work.

[RobbieTheWagner]

@FalkF Thanks for the PR! We've recently made a lot of significant changes to ember-light-table. Can you please rebase your PR and make sure everything is still working as intended? If this PR is no longer necessary after the new changes, please close it. Thanks!