[BUGFIX] Test loginUser before frontendUserRepository

Prevents a PHP warning if $GLOBALS['TSFE']->fe_user->user is not set
[BUGFIX] Wrong condition for checking Url

to check if the site URL is not already the first part
the condition should be 0 !== strpos....

Conflicts:
	Classes/ViewHelpers/Page/AbsoluteUrlViewHelper.php
[DOC] Fix small typos in comments
[BUGFIX] Ensure unique ids of nested menus
[BUGFIX] Avoid path conflicts on CSS assets

When copying files referenced inside CSS files (the "rewrite" process),
only the filename of the examined CSS file is taken into account for
obtaining a temporary name. That leads to name conflicts if two CSS
files both reference to a – different – file that is reachable via the
same relative path from within each of the two CSS files.

Conflicts:
	Classes/Service/AssetService.php