#12077 Add support for download attribute to link plugin

This change adds a checkbox to set the HTML5 download attribute in the advanced tab of the link dialog, allowing to force a download.

Please note that setting a custom file name for the download as mentioned in the comments of the related ticket was not implemented this time. Having two fields (checkbox + textfield) managing one single attribute would have led to a much more complicated change.

![download-flag](https://cloud.githubusercontent.com/assets/7569142/4335875/293748da-4001-11e4-9a5a-28d4384ee2d7.png)