Expose colorscheme, scrollbar, clipboard functionality to QML

[gber]

I am working on modernizing lomiri-terminal-app which is a continuation of the ubuntu-terminal-app from Ubuntu Touch and now part of the Lomiri project. ubuntu-terminal-app is built on a heavily modified, vendored fork of qmltermwidget from the lxqt project. Rather than maintaining this in parallel we'd rather prefer to join forces, particularly since this fork of qmltermwidget is already packaged in major distributions. This PR exposes some additional functionality to QML in order to make lomiri-terminal-app work with qmltermwidget. The most important change is the exposition of ColorSchemeManager and ColorScheme since we provide a QML-based color scheme editor as part of our settings dialogue. We are also using our own scrollbar based on our own QML components library. Finally, we need some additional clipboard/selection functionality in order to show popovers on paste.