git-gui: reduce Tcl version requirement from 8.6 to 8.5

git-gui: reduce Tcl version requirement from 8.6 to 8.5

On some MacOS distributions like High Sierra, Tcl 8.5 is shipped by
default. This makes git-gui error out at startup because of the version
mismatch.

The only part that requires Tcl 8.6 is SimpleChord, which depends on
TclOO. So, don't use it and use our homegrown class.tcl instead.

This means some slight syntax changes. Since class.tcl doesn't have an
"unknown" method like TclOO does, we can't just call '$note', but have
to use '$note activate' instead. The constructor now needs a proper
namespace qualifier. Update the documentation to reflect the new syntax.

As of now, the only part of git-gui that needs Tcl 8.5 is a call to
'apply' in lib/index.tcl::lambda. Keep using it until someone shows up
shouting that their OS ships with 8.4 only. Then we would have to look
into implementing it in pure Tcl.

Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>