how to open sources from preview

[AlexTugarev]

if you double-click open .plantuml file from navigator, and the preview widget is preferred due to theia settings, it's not easy to find the way to the source. currently one need to choose Code Editor from the context menu in the navigator.

it would be better to see some kind of overlay button to open the sources.

the double-click event is used for toggling scale-to-fit.

[AlexTugarev]

WDYT @marcdumais-work ?

[marcdumais-work]

+1 - such an overlay button in the preview widget would be nice.