Inspector: rename ProbesDataGrid to ProbeGroupDataGrid.
Inspector: move rules from ProbeGroupObject.css into ProbesSidebarPanel.css.
Inspector: simplify the style rules for probe group data grid, and fix some
minor bugs with the probe group gutter marker.
Add a hash implementation for TextPosition.
Keep track of probes in the backend by line+column, instead of just line number.
Inspector: make column numbers mandatory for the protocol's ScriptProbe object.
Inspector: linkify the probe locations displayed in ProbeGroupDetailsSection header.

 * store raw probe source locations with a SourceCodePosition rather than ints
 * when presenting location information to the user, use a SourceCodeLocation
 * create new probes using raw line and column numbers