Open the issue tag list on click

see #4247

Now the list of available tags will open as soon as i click on the label field.
Previously i needed to type chars of existing tag(s) to open the list.

**Old:**
![old_new](https://f.cloud.github.com/assets/327488/629108/c11cadf2-d126-11e2-8bc7-065ae08cfe67.gif)

**New:**
![new_new](https://f.cloud.github.com/assets/327488/629109/c77e179e-d126-11e2-9da1-5945ce643f15.gif)