Selections in drag mode now follow the following rules:
* If a line is selected in the grid, at least one visual feature
corresponding to the line is selected.
* If a line has any features selected, that line is selected in the
grid.
In addition, all control points now start out selected in the vector
clip tool, and all tools should no longer discard the current selection
at unpredictable or unintended times.
Updates #513.
Originally committed to SVN as r4363.