Allow focus on the Qt render widget

By default widgets are set to the focus policy Qt::NoFocus which disallows manually focusing it. Changing the policy to allow clicking the widget to set focus to it allows for keyboard input when not rendering to a popout window. This commit also sets focus to the widget when showing it.

Fixes issue #158.
Allow focus only when in popout mode

Only allow manually setting focus to the rendering widget when in Single Window mode. Apply this behavior to when changing the mode while an app is running.
Remove duplicate work
Merge pull request #344 from Yllodra/Qt-Oddities

Allow focus on the Qt render widget