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ARIMA and VAR widgets were somewhat outdated, gui-wise.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

[codecov-commenter]

Codecov Report

Base: 73.93% // Head: 73.93% // No change to project coverage :thumbsup:

Coverage data is based on head (4157c13) compared to base (4157c13). Patch has no changes to coverable lines.

:exclamation: Current head 4157c13 differs from pull request most recent head fcdd0c3. Consider uploading reports for the commit fcdd0c3 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #235 +/- ## ======================================= Coverage 73.93% 73.93% ======================================= Files 28 28 Lines 4305 4305 Branches 582 582 ======================================= Hits 3183 3183 Misses 996 996 Partials 126 126 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=biolab). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=biolab)

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