(ENHANCE) Port the 'Check Out' dialog to Ttk

The selector to the right will have to wait until that mega widget
is ported.
(ENHANCE) Port the 'Check In' dialog to Ttk
(BUGFIX) Changed the discard command for rcs to 'co -f -r'

Used to be set as 'unco' which a haven't been able to find in
any rcs docs, maybe it used to be a common to create it as
script or something.

It probably means that noone has used this option for ages.
(BUGFIX) Fix some syntax that used to be ok in former versions of Tcl

It used to say:

if {[catch [eval exec -- ${cmd}] msg]}

But with 8.4 it had to be changed to

if {[catch {eval exec -- ${cmd}} msg]}

or it would throw a fit
Merge branch '8.4' into Ttk

Conflicts:
	CHANGELOG