Added quickfix framework

Added frontend logic and data structure for quickfixes. 
A language-support extension can now add MarkerResolutions to the generated markers in analyze(). These resolutions are then shown in the editor by clicking on the error/warning icon on the gutter or via the key shortcut (ctrl+shift+q). They can be applied by selecting the respective item from the popup.
For an example of how MarkerResolutions are generated, see our xquery extension:
 https://github.com/wcandillon/cloud9/blob/master/plugins-client/ext.xquery/quickfix/XQueryResolver.js