Add Dark mode support

[miskovoj]

Adds full support for dark mode:

• New stylesheets and icons
• Automatic detection of current theme (light/dark)
• Palette update propagation to user widgets

Changes:

• For each stylesheet and each icon file a dark variant was added with _dark suffix.
• Stylesheets were adopted for dark mode while maximizing usage of palette colors
• CDockOverlay adjusted to use palette colors only and to get updated when application palette changes
• A setting for stylesheet switching behavior was added to CDockManager offering three options: automatic switching according to application palette and fixed light or dark mode, which follow the current behavior. The automatic setting is default while the fixed modes are designed for manual switching, custom palettes or possible compatibility issues.
• (Unrelated fix) Tab menu icon was added to focus-highlighting stylesheets

#753