Make opening of the optional feature form modal, with auto-deleting of feature on feature form Cancel

- Note: deletion on Cancel is same behavior as main digitizing tools
- Add property to ignore clicks when tool is active, keeping the tool from registering clicks when they occur outside of a modal dialog
Fix tool not switching to correct geometry when switching between layers in edit mode
Add some transparency to tool's rubberband and set color for its polygon similar to that of main digitizing tools
Merge pull request #2 from dakcarto/update-for-2.0

Further updates for QGIS 2.0