refactor: qml gui

[RobBuchananCompPhys]

Beginning of a refactor to create a fully QML-based GUI that can be built using a feature flag in CMake. Aims to get a QML Dissolve main window as a starting point to experiment with redesign.

We may eventually need to break out the Qt Models directory (currently in gui/models) and the QML types register into a destination where it can be accessed by both GUI and QML builds.

[rprospero]

One thing that comes to mind is that it might be better to make the
QML version truly its own target and executable, instead of just being
a switch that gets pulled inside dissolve-gui.cpp.

Advantages

• No extra preprocessor macro logic in the executable
• No CMake flag required (just a new target).
• Easier to make independent changes to each program. This will become important when we start including models to go with the GUI.
• The CI can compile both executables in one pass and ensure that nothing has broken in either GUI.

Disadvantages

• Still some duplicate code (e.g. CLI argument handling) that would need to be maintained in both executables
• I have no idea what to name the new GUI

[trisyoungs]

I definitely think the separate executable, as Adam suggested, is the way to go, and have it configurable at build time with a CMake option perhaps?

Bit of duplicate code I can live with for now, but as for the name.... maybe dissolve-gui-qml?