Code clean-up

Fixed potential bug in `ScriptManager.addScript` where frontend url was used
instead of debugger script name.

Fixed potential bug in `breakEvent` where "go to location" breakpoint
was cleared even if we hit a breakpoint in a hidden file. This event
sends `continue` request immediately, thus the program would never pause
as the result.

Simplified `breakEvent` and removed script source fetching, since we are
already sending new scripts to the frontend as soon as they are loaded.

Removed `includeSource: true` from the debugger request used to load scripts,
since the frontend always sends a standalone request to get script source code.

Moved Agent registration from session.js to FrontendCommandHandler.

Replaced usage of debugger.request() with sendDebugRequest() and renamed
request to _request. There are stil two places where debugger.request is used,
but those are old functions that are never executed.
(We will remove or rewrite them later).

Wrapped lines longer than 80 characters.