Enhancements to local tasks

Note: this is the same issue as #2458739: Enhancements to local tasks, just for the 7.x-2.x branch

This is purely a user interface improvement.

The "Print" and "Email" links (local tasks) are renamed to "Printable invoice" and "Email the invoice".
Also, default local tasks for "View" and "Edit" have been added, to make it more clear on which page is the user.

Here are the screenshots:

Before: http://i.imgur.com/43hVT9y.png
After: http://i.imgur.com/yQwb8Df.png

This has no impact to any other function of the module, so I believe it is safe to commit it directly to both branches.

Commits will be posted shortly. The issue is here only for future reference.