Add a Browser service

It's nice to be able to browse an object's class by doing `aThing browse`
in a workspace. Creating a method that opens the Helios browser is very
easy, but then Object relies generally on Helios, and specifically on the
current implementation of Helios.

A Browser service has been created, and anything that considers itself a
browser can register. Just make sure your browser responds to `open` and
`openOn:`. Objects can be browsed by telling them to `browse`.

The Helios Browser and older IDE Browser will register themselves, with
the IDE Browser deferring to Helios if it exists.
Fix a typo in a comment about IDEBrowser

I incorrectly indicated that an IDEBrowser should inspect a class. It
should never `inspect` anything, it should just open itself upon that
class.
Don't get a debugger when browsing

When you press the BrowseIt button you should not get a debugger; instead,
you'll end up just browsing Object.
Find references to a thing if browsing fails

The Browser should try and `DoIt` a line or selection before browsing the
returned object's class. If an error occurs, then the line probably wasn't
valid Smalltalk, and may be a text snippet, so a References browser is
opened.

Behavior is undefined if your code contains an underscore (_) that isn't
in a string or comment.
Bind browsing to a key command

You can browse selected text in an editor by pressing the `BrowseIt`
button, or by pressing `Cmd-B` on a Mac or `Ctrl-B` on a PC.
Activate items in PackagesListWidget

Items in `PackagesListWidget` will be activated when selected via
`HLBrowserModel#selectedPackage:`, even if the list doesn't have focus.