[patch v2] ide: add hide line number feature

This feature is available into the editor/display section of the
preference dialog.

This is the second version of this following pull request: https://github.com/supercollider/supercollider/pull/1121

changelog since v1:
  - use spaces and no tabs
  - set show line number as default value
  - access the feature using the preferences dialog instead of the menu bar