[1.0.0-alpha2] Remove SlotSourceProxy

Problem/Motivation

Like ui_patterns_overrides sub module, SlotSourceProxy is a temporary class which must be removed once UI Patterns has implemented mechanisms covering the 3 methods:

• ::getLabel()
• ::getLabelFromData()
• ::getLabelWithSummary() once 📌 [1.0.0-alpha1] Use PluginSettingsInterface::settingsSummary() Active is merged

Proposed resolution

Move logic to UI Patterns 2. It will not be trivial to known how and where it will land, but we can ask Christian & Mikael.

Don't move the premature optimization ($this->memoryCache) because:

• it add complexity and efficiency was not proved AFAIK
• let's trust the UI Patterns 2 cache & perf strategies