Progress indicators

[uliwitness]

We need a progress indicator part, with both indeterminate spinner, indeterminate bar and determinate bar states. Maybe add a determinate spinner that is essentially a pie chart (maybe like iOS download progress) for symmetry.

Again, should default to standard look but be themeable with icons if desired.

[uliwitness]

I'd call this an advanced starter bug as it involves defining a new type of part, but since that would basically mean copying and pasting e.g. the code for buttons in the parser (Forge), interpreter (Leonie), CLayer and defining a new CButtonPart-like class, then implementing the actual Mac-specific cocoa views (like CButtonPart already does, just for an NSProgressIndicator), this is a good way to get to know the entire codebase in a comparatively painless way. Maybe one could split that up as well. Someone creates a CProgressIndicatorPart, and a beginner then fills out the Cocoa stuff and bridges the properties and tests it.