Use groups in Pattern presets library

Problem/Motivation

Pattern presets config entities have a "group" property which is a simple (translatable?) string:

id: 68679ab4e1797
label: "My button with a title"
description: ""
group: ""
theme: usb_sub
sources: [...]

But it is not used anywhere.

Proposed resolution

Among the patterns showing up in the Pattern Libray panel (not all patterns are displayed here all the time, because of 📌 Context & dependency management for Pattern presets Active ):

• group them by group. Patterns without groups go to "Others"
• if there is only group after the grouping, flat the list instead of keeping the group