API review and codebase improvements

[blammit]

After the 1.0.0 release it would be nice to clean up the API to make it more clear and improve ease of use. Also there are various implementation patterns/concepts that can be improved. For example:

Larger issues to resolve:

• Rework Device List model and delegates #1339
• Generalize the code for opening pages based on service type #1388
• Don't create data objects unnecessarily #1400
• Provide VisibleItemModel equivalent that filters out non-visible items #1732
• Avoid rebuilding models for ListQuantityGroup and ListTextGroup  #1338

Naming:

• ListItem-derived components are named inconsistently. E.g. we currently have ListNavigationItem, ListAlarm, AlarmLevelRadioButtonGroup. Clean up naming of ListItem-derived types #1760
• Rename VeQuickItem isValid -> valid to avoid porting errors and inconsistent property naming
• Rename ListItem allowed to something else. We want something like visible but named differently (show is less preferable as that sounds like a function). (Or maybe it can be removed altogether?)

Code style:

• Review code style and maybe run clang-format and QML equivalent on git commit

Improving UI controls:

• Ensure controls always reflect the backend data state. We can show some intermediate state that indicates the control has been pressed, e.g. for toggle switch shows a busy spinner or slides to the middle or such.

Generic improvements:

• Some verbose patterns are duplicated (e.g. bottomContentChildren: [ ListLabel { ... } ] combo)
• Theme value consolidation - there are way too many values, should use more generic values
• Global.dcInputs is used for both input and output services
• Have base classes for UI components or models where necessary
• Rework OverviewPage to avoid the needs-relayout-retrigger
• Use typed properties instead of var everywhere that it is possible. Helps with tooling (e.g. IDE auto-complete) and also possibly a performance improvement when accessing the Global properties.
• Utils.js: remove unused functions and move features to C++ where it would be useful