Impove opera blank drag image workaround (stolen from codemirror)
Fix issue with blinking cursor on drag over

This happens when dragging from another app. Editor can get focused on app icon hovering (on a Windows taskbar).
Change mouse capture target container -> content

To prevent hover activity of scrollbars in Firefox
Merge pull request #1661 from danyaPostfactum/dnd-fixes

Some drag'n'drop fixes