new attribute: link open how

[wordmuse]

When you want to follow a link the three choices currently available are open in same tab, open in new tab, and open external. The Link Open How attribute would let me set any of these as the default for an item or set of selected items.

I'd also like keyboard+mouse combinations for these three choices if no attribute is specified. I'm thinking:

- click - opens in same tab
- ctrl+click - opens in new tab
- alt+click - opens external

When URP eventually supports multiple windows, then perhaps additional keyboard+mouse operations could be created too?

- Bal