Qt Bridge: Add support for designing runtime-switchable themes

The current mapping of Figma Variants to QML States make it easy to change the appearance of individual controls (e.g. Hovered vs. Selected vs. Normal vs. Disabled). It would be good to also have a mechanism for implement a system-wide theme change that can be triggered at runtime (for example: switching between Light Mode and Dark Mode – this is also listed as a use-case for Figma Variants at https://help.figma.com/hc/en-us/articles/360056440594-Create-and-use-variants )

Perhaps we could mimic some Qt Quick Controls Styles:

• The Imagine style lets us change a global path property which then loads assets from a different directory (https://doc.qt.io/qt-6/qtquickcontrols2-imagine.html#path )
• The Material and Universal styles let us change a global theme enum property