Fix various Windows-issues

File handling was broken on Windows. This commit contains a revision of the path handling with separation of file paths and urls where needed.

There may be remaining issues and there may be better ways to do this, but it is easier to start that refactoring job with a set of passing tests.

Fixes #687
Fixes #660
ERROR-log on symbolic links

filepath.Walk does not follow symbolic links.
There's no easy fix for that outside of Go, so the best we can do for now is to give notice to the end user by ERROR log statements.

This commit also fixes a related panic situation in GenerateTemplateNameFrom when the layout dir was a symbolic link.

Fixes #283
Make Urls in menus respect CanonifyUrls

Fixes #519
Revert "Make Urls in menus respect CanonifyUrls"

This reverts commit 3ccb3979028aafb3e6cc16d47daa77a1fdea11fa.