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).