IDEView: set active pane state [#80356544]

Set focus state of IDE when window loses and gains focus, user switches panes, tabs and apps.

[Focus is not preserved when app, pane and tab is switched](https://www.pivotaltracker.com/story/show/80356544)

Placeholder panes are bare `KDCustomHTMLView` instances. They don't have `setFocus` method as a property.