Ability to have different KirigamiThemes in the same process

This is intended to use primarily in Plasma. It adds the ability to
define a custom theme for areas of the application based on a property
on the relevant QQmlContext (a QProperty on the context, *not* a
contextProperty, so it's accessible only on the c++ side).

The goal is to allow usage of Kirigami.Theme and Kirigami.Units inside
plasmoids. They will then use the KirigamiPlasmaTheme instance, while
everything outside, such as config dialogs will use the org.kde.desktop
style.

This will unlock the ability to remove any components in PlasmaCore,
PlasmaComponents, and PlasmaExtras that don't use any SVG styling and
simply duplicate their Kirigami counterparts, with the only difference
being that they get their colors from the Plasma theme, rather than the
desktop theme. Such functionality will no longer be needed, and we can
use the Kirigami versions with no issues.