qtui: Add keyboard/mouse access to version links

qtui: Add keyboard/mouse access to version links

Enable the use of mouse and keyboard for accessing Quassel version
information links, including mouse selection (for copy-paste).

This fixes having to right-click -> Copy Link Location to open commit
information.  Note that untrusted clients/cores can set their version
information to open ANY external link, including disguised/phishing
links.  However, this requires manual interaction and I suspect
there's bigger problems if one is connecting to untrusted Quassel
clients/cores and clicking the links in File -> Core Info...

Update About to also allow keyboard access to the version link.  All
the other links were keyboard accessible already.