Add ctrl/shift key modifiers for link click

* ctrl+click = open in new background tab
* shift+click = open in new tab

I'm not sure if this is the best way to do this. I originally looked to use emit requestedOpenUrl, like the middle click option, but couldn't find a way to do open link in new (non-background) tab using that. I'm also not sure if it's safe to touch m_hitResult from here. Please verify that this should be okay before merging.