Extension Manager: (automatic) code style fixes

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.

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First start at refactoring the extension manager

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