Enable tab dragging in RubyMine

Since RubyMine 3.1 you can drag tabs across panes/windows and out of the main window to create new windows.

For any version below 3.1 do it like this (will only allow dragging tabs inside their pane, not across panes):

1. File → Settings
2. Editor → Editor Tabs
3. Check "Show tabs in single row"

Seriously.