Update trumbowyg.js - add predefined targets for anchor elements

[coelhudo]

In the current version, when a link is inserted, that's the modal dialog that shows up:

The target is a text field. However, the target attribute only accepts four keywords: _self, _blank, _parent, and _top.

I changed the code to use a drop-down menu instead of a text field, so the users don't need to type the value.

[Alex-D]

Thank you! :) I will review that soon, seems good, but I need to try it for real, and maybe make option labels translated

[Alex-D]

Merged:

• 2cf579774ced60bc0abc00ef16b484958c037902
• 63b64fae8f838709dfba4ea4e063beb5659fe081 (some additions/fix)

Thank you :)