show empty tag values in multi-selections

[k-yle]

Closes #9876

If you select a node with natural=* and another node with no natural tag, the multiselect dropdown now includes (empty) as an option, to avoid confusion.

[tyrasd]

Thanks. I've made a few minor tweaks:

• use italic font style to make option visibly different from regular tag values
• switch back to use the raw tag value as the displayed option label in the combobox dropdown: using d.title was confusing for tags where taginfo returns rather long descriptions (these are still displayed as the respective option's hover text, btw)