LinkGenerator: extract interface

[PavelJurasek]

• feature
• BC break? no
• doc PR: should add note to https://github.com/nette/docs/blob/doc-3.1/en/creating-links.texy?

Since LinkGenerator is final, it would be nice if it had its own interface so that it can be replaced with own implementation.

[dg]

What is the interface for? Replace it with your own implementation.

[PavelJurasek]

Right, I can change service application.linkGenerator to whatever class I want, but it won't solve the problem with 3rd party packages that are typehinted to Nette\Application\LinkGenerator. That leads me to conclusion, that the PR should be reworked to:

• create ILinkGenerator interface
• create alias LinkGenerator for ILinkGenerator to keep BC
• rename current class implementation, DefaultLinkGenerator or similar

[dg]

@PavelJurasek I'll save this as a beautiful argument when someone wonders why I'm removing the letter I from the interface names. :-)