This (partially) reverts commit 127bbdf653.
Only incorrect changes from Kirigami.Units.largeSpacing to
Kirigami.Units.gridUnit are reverted. Only Plasma.Units were buggy.
Move from a C++ library + QML plugin to a QML plugin only for simplicity, since the homescreen switching architecture will be done from Plasma, and so use of the shell library only needs to be from QML.