Mechanism to use own Kirigami::Theme in plasmoids

AppletContext now has a _kirigamiTheme property which will be used
by Kirigami to load a different plugin when asked.

This adds a C++ based platformTheme to kirigami which uses Plasma::Theme
colors when passed a Kirigami::Theme instance, and similarly for Units.

This will make possible to get completely rid of PlasmaCore.ColorScope
class also should be possible to remove the QML binding of Plasma::Theme
in PlasmaCore.

Requires frameworks/kirigami!1120.

This can be tested by, for example, replacing instances of
`PlasmaExtras.Heading` with `Kirigami.Heading` in Plasmoids. When using
the Breeze Dark Plasma theme and the Breeze Light color scheme, the
Kirigami headings should now get their the colors from the Plasma theme
and remain visible, rather than getting their colors from the systemwide
color scheme and turning invisible.