Start implementing properties and file attributes.

Add test, test data and rigging. Not implemented yet.
Implement FileAttributeLine.

It represents a line that holds a file attribute.
Implement basic parsing of file attributes.
Also allow empty but properly named file attributes.
Verify not finding non-existent file attributes.
Implement parsing file-level attributes. Extend test.
Implement rump of Drawer class.
Implement AttributeLine base class for attributes.
Implement DrawerEntry as AttributeLine.
Start implementing drawer parsing.
Rigging for the first test of two-pass parsing.

Of course the test fails so far.
Implement first parse pass that identifies file properties.
Separate standard syntax from tests for corner cases.

Add drawer closing entry structure.