Enable use of any Xcode, anywhere

These changes revoke the assumption (really, assumptions, plural) that Xcode is necessarily either in /Applications or /Developer/Applications.

Included are UI changes in both the Dev Tools Path dialog and the Preferences window to abolish the old pair of radio buttons—since it isn't a binary choice anymore—and changes in the code to end the conditional disablement of the path controls.

Also included is a change to one of the preference defaults: AppKiDo now defaults to using the user's blessed Xcode, not the increasingly-archaic /Developer. This is done using the previously-unused `+[AKDevToolsUtils devToolsPathAccordingToXcodeSelect]`.

Finally, the Open panel for selecting a Developer directory now accepts Xcode.app bundles. If the user chooses an Xcode, this implementation translates that to the Developer directory therein (if there is one).