[NOMERGE YET] ARIMA, Var: Minor refactoring and GUI update

[janezd]

Issue

Reapplication of #200 .

ARIMA and VAR widgets were somewhat outdated, gui-wise.

Requires https://github.com/biolab/orange-widget-base/pull/202 to be released

Description of changes

Use WidgetPreview and deferred apply button. Also use QGridLayout.

I removed hints that belong to documentation.

Includes

• [X] Code changes