Editor: restore document panes when resetting a layout

[ivan-mogilko]

"Reset layout" command is commonly used when user did a mistake and moved panes into wrong positions.

But when executed, this command closes all document panes (editors, previews, etc). This may be annoying to users.

A proposal is to try to restore previously opened "content documents" after layout reset. To clarify: this refers only to the panels which are opened by clicking on project explorer items.