Style configuration: return ModuleView from module views (refactor) #1023

Part of the style configuration plan in #1022 (from #925 by @sulabi).

Goal

Modules' view() returns an iced::Element, so once a module's row of elements is built, its children can no longer be styled one by one. grouping = "individual" (#1022) needs exactly that: give each element of a module (e.g. each SystemInfo indicator) its own island.

#925 solves this with a ModuleView type that keeps a module's elements separate until the bar applies the style, and only then turns them into an Element.

Scope

• Bring the ModuleView refactor from Allow for more style configuration #925 onto the current main: modules return ModuleView, and src/modules/mod.rs builds the module items from it.
• No visible change: the bar must look and behave exactly as before. No config options are read yet.

@sulabi, since this is your design, feel free to pick it up if you'd like to.