[BUGFIX] Several typos in Page Browsing ViewHelper

Fixing several typos and slips of the pen in the
PageBrowsing ViewHelper.

Without fixing all of them the page browsing ViewHelper
is not functional properly, that´s why i want to submit
them at once and also supply the fix for it.

That are these errors:
* Undefined but used class property prefixId
* Overwriting instead of appending content to already defined variable
* Not using UpperCamelCase for extensionName in Localization Utilty
* Accidentally using wrong variables
* Defining variable in for-loop which could be defined outside also
* Using undefined variable freeIndexUid

Resolves: #55340
Releases: 6.2, 6.1
Change-Id: I708a7af4876eba0e69fa666694315e0babcf6800
Reviewed-on: https://review.typo3.org/28539
Reviewed-by: Markus Klein
Tested-by: Markus Klein