Rework implementation of field type icon [#3443904]

Problem/Motivation

Currently the field type icon for the view mode switch field type is added by a custom hook_library_info_alter() implementation. But core (>= 10.2) allows to add it via hook_field_type_category_info_alter() as well.

Steps to reproduce

n/a

Proposed resolution

Use general instead of Layout as category for view mode switch field type
Rework the custom implementation and use hook_field_type_category_info_alter() instead

Remaining tasks

Create issue fork and MR to fix this issue

User interface changes

n/a

API changes

n/a

Data model changes

n/a

Issue fork view_mode_switch-3443904

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3443904-rework-implementation-of changes, plain diff MR !34
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