Get rid of actors from GamePlugin.

We don't get much benefits from using actors in GamePlugin implementation.
Using Future is more functional and eaiser to compose.

This patch broke JavaScript timer functions such as setInterval and setTimer.
A follow-up patch will fix this.
Get rid of JavaScript timer functions.

setTimer and setInterval are rarely used. Actually, these functions should not
be used in handler functions because scheduling multiple timers at the same
time has a negative impact on performance. Because the philosophy of Beyond is
to systematically discourage bad programming idioms, I think it's better to get
rid of JavaScript timer functions entirely.
Remove timer function names from BeyondGlobal.

Forgot to remove timer function names in
aec99b65d7a38b9b3fc11cc39adcabee848ee9ee.
Remove JavaScriptConsoleProvider.

JavaScriptConsoleProvider unnecessarily complicates code. Use a unified
implementation in ScriptableConsole and get rid of JavaScriptConsoleProvider.
Extract scriptableClasses an an array.

Don't repeat ScriptableObject.defineClass for each sciptable class. Extract
scripable classes as an array and call ScriptableObject.defineClass for each
class.
Change ScriptableFuture.executeCallback to take variable number of arguments.

This simplfies code because we don't need to create a JSArray when calling
ScriptableFuture.executeCallback.
Don't repeat callbackOnFailure twice.

Move callbackOnFailure to the outer block so that two error cases can share the same
callbackOnFailure. Also move callbackOnSucess for consistency.
Use _ for unused argument name.