Partial icon improvements

These are partial improvements ready for merging with 5.0 branch. The outstanding work depends on VDG team's delivery of missing breeze icons (https://bugs.kde.org/show_bug.cgi?id=421382).

This change:

1. Simplifies icon mapping code and prepares for upcoming breeze theme delivery:
   • Breeze is the mandatory theme on macOS and Windows, so make it explicit
   • setThemeName() is only supported on non macOS/Windows platforms
   • Add a single iconMappings container for all Icons::Icon enums to theme mappings
   • remove the KDE icons generalization – replace it with distinct Oxygen and Breeze mappings.
   • simplify the convoluted setIconThemeNames() mapping logic
2. Continues the refactoring of icon names to reflect the context of usage.
3. Fixes some old filename typos.
4. Replaces the explicit QIcon::fromTheme() leftover calls with Icons::get() wrapper.