editor: use QKeySequence::StandardKey

Each target-platform has it's own set of default key-bindings. Use
QKeySequence::StandardKey to get the OS-default behaviour.
editor: use icons from theme, if available
editor: use Qt's built-in window-title management

This allows each platform to get the title styled in it's
traditional way.

Unfortunately, we also need to work around QTBUG-16507 for this
to work properly.