Add `QtInstallDir` as equivalent to `GNUInstallDir`

It would be nice to expose some standardized interface for how we expect a user Qt project to install its artifacts to. For example, `kirigami` installs its qml files in the same path as Qt's files, but this is defined manually via `KDE_INSTALL_QMLDIR`. This could be instead more standardized for simple projects to pick up the default install paths as defined by the OS.

One potential challenge to this, is that when you run `include(GNUInstallDir)`, the `install(DESTIONATION)` are automatically populated, but would we be able to provide a similar user experience?