Fast access to basic properties

[emersion]

Have a list of basic properties and provide fast access to them. These are:

• CRTC_ID
• FB_ID
• CRTC_X, CRTC_Y, CRTC_W, CRTC_H
• zpos

[emersion]

Some work for this has been done in the basic-props branch. Not yet merged because this doesn't appear to be a bottleneck (string comparisons and list traversals for property lookup seem fast enough for now from a perf report). Maybe I'll merge this anyway because we'll want to fix it eventually.