textproc/tkdiff: port improvements

• add DESKTOP_ENTRY
• add converters/base64 as BUILD_DEPENDS
• add icons

Also give maintainership to portmaster@bsdforge.com

PR: 228176
Submitted by: portmaster@bsdforge.com
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D15480