Add beginnings of sqlalchemy based storage
Create a couple of shims to allow the new db connection to be used with old table style
Switch to declarative sqlalchemy models
Start converting layer functionality to use models
Add point and segment models
Layer table model now uses Layer objects directly
Remove leftovers from Layer TreeWidget
Add document settings table to store active layer
If active_layer not in settings then set it to first visible layer when opening a file
Fix toggling of visibility via the checkbox
Minor edits (typos, whitespace, etc)
Add circle and ellipse model
Rename CadWindow->MainWindow, IDocument->SubWindow
Refactor out common table parts, add generic foreignkey code
New command handling via signals
Add updated circle/segment commands
Update point to use new style commands
Add new ellipse command/preview/entity
Add circle preview file (was never added)
Update rectangle command to work with new commands