Fix styling issue with the modify rectangle.

[jlap]

This commit fixes a glitch with the modify rectangle that only occurs on the built version, and it also fixes a the styling issue in the gmf draw feature example where selecting the rectangle causes the style to be added twice.

This was occurring by a combination of the modify rectangle wanting to manage its own layer of polygons, which really wasn't necessary, and a bad management of added / removed features when creating / removing their corner handles.

The example now working well in the integrated draw feature example is available here: http://jlap.github.io/ngeo/modify-rectangle-style-bug/examples/contribs/gmf/drawfeature.html

The modify rectangle works exactly the same as it used to: http://jlap.github.io/ngeo/modify-rectangle-style-bug/examples/modifyrectangle.html

[jlap]

@fredj @fgravin this is ready for review.

[fredj]

Looks good, thanks