Don't complain about changed download URLs when plugin repository is unavailable

Fixes #4459

Extension Manager: correctly handle component types

There were a few missing and they were also missing from the detail
list. Searching for components using the type: prefix now works.

Extension Manager: check dependencies on uninstall/disable

Extensions that others depend on should not be uninstalled.

Extension Manager: protect the current template

Extension Manager: fullpath instead of realpath

This should fix backslash problems on Windows and the various additional
checks and ommissions our implementation does make more sense here
anyway.

Extension Manager: (automatic) code style fixes

Extension Manager: updated tests

Extension Manager: adjust javascript and images

This simpliefies the view options into a single filter, which toggles
between all and updatable extensions. This should be less confusing and
is what

Extension Manager: adjust javascript and images

This simpliefies the view options into a single filter, which toggles
between all and updatable extensions. This should be less confusing and
is what most people use anyway.

Image use has been reduced and SVGs are used for icons now. Placeholders
are still PNG.

show more ...

Extension Manager: Template GUI

template tab works now, fixed installed status

Extension Manager: First go at reimplementing the GUI

still a long road ahead

Extension Manager: ensure php requirements before installing

This makes use of the new minphp and maxphp fields in the info.txt

Extension Manager: CLI now should completely use new classes

Extension Manager: make installation work again

The Installer class now also handles dependencies

Extension Manager: more refactoring

started to migrate the CLI over to the new classes

ExtensionManager: allow initialization using an ID only

This is a more convenient method

First start at refactoring the extension manager

The aim is to readd the existing functionality into proper classes, then
extend from there.