Manifest should be resolved lazy

[martenb]

We have problem in tests, because Nette App is failing, if manifest file does not exists, even, when it is not needed.
So i think, better solution would be, if manifest is loaded, only when it is needed.

[f3l1x]

Yep, agreed. Can you prepare PR?

[lubomirblazekcz]

I plan to look at this, the manifest resolving could be much easier and this could be improved too.

But I am currently on vacation, so I will have time for this in a few weeks.