[FIX] Ensure visible nodes after openning a workflow.

[ales-erjavec]

Issue

In case the user opens a workflow whose nodes are very far from coordinate origin there is nothing visible in the viewport.

Example:

<?xml version='1.0' encoding='utf-8'?>
<scheme description="" title="" version="2.0">
	<nodes>
		<node id="0" name="Constant" position="(3779.0, 2765.0)" project_name="Orange3" qualified_name="Orange.widgets.model.owconstant.OWConstant" title="Constant" version="" />
	</nodes>
	<links />
	<annotations />
	<thumbnail />
</scheme>

Description of changes

Scroll the viewport to the first node.

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #2490 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2490      +/-   ##
==========================================
- Coverage   74.52%   74.49%   -0.03%     
==========================================
  Files         321      321              
  Lines       56086    56086              
==========================================
- Hits        41797    41781      -16     
- Misses      14289    14305      +16