[TASK] Cleanup update wizard in Install Tool

Cleanup the update wizard classes and move all
classes to one folder instead of the two that
are used now.

Change-Id: Id6a000fe75892d1596984954b2b2f8a78eaf0483
Resolves: #50670
Releases: 6.2
Reviewed-on: https://review.typo3.org/22754
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
[BUGFIX] Fatal error: "enableFields on non-object" in extension manager

Don't rely on $TSFE object to detect if we are in a frontend context. Rely
on TYPO3_MODE constant instead.

Releases: 6.2, 6.1, 6.0
Resolves: #50803
Change-Id: I06ea2692dab7683c7284e5f3ff45036d72c64999
Reviewed-on: https://review.typo3.org/22830
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
[BUGFIX] Fix empty href parameter

In case the link target is not available (maybe hidden), the behaviour
known from the Core is to render the link text, but no a tag. The pageVH
renders the link, but the href parameter will be empty.

Resolves: #50751
Releases: 6.2, 6.1, 6.0
Change-Id: I526aa0e3d42f8aba867b3f996bac2366dc1f6d19
Reviewed-on: https://review.typo3.org/22789
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters