drag feature: incoherent cursor style

[fredj]

To reproduce:

•  http://www.openlayers.org/dev/examples/drag-feature.html
• draw a polygon
• select "drag feature" and move the cursor into the feature (the cursor style in move)
• drag the feature
• error: the cursor style is default but it should be move because the cursor is above the feature

[fredj]

set the cursor style to 'move' if the cursor is above the feature (this.over)

[elemoine]

patch-1673-r7712-A0.diff includes an additional comment and unit tests.

[elemoine]

Unit tests pass on FF2, FF3 and IE7. Fredj, please commit.

[fredj]

(In [7713]) keep the cursor style to 'move' if the cursor is above a feature. r=elemoine (closes #1673)