Save-button should be disabled (greyed out), if no changes were made since the last save

**Mikhail T.** ([Bug 53857](https://bz.apache.org/bugzilla//show_bug.cgi?id=53857&redirect=false)):
Subject says it all, really... By accepted standard of the GUIs, the "Save" button (the one still showing the 3" diskette no one uses any longer :-) should be disabled, when the most recently saved version is still current.

OS: FreeBSD

Start by locating the JMeter GUI Save action and the state that tracks whether the current version has changed since the last save. Reproduce the issue by opening a saved item, making no changes, and saving it; done means the Save button is visibly disabled when the saved version remains current and becomes enabled after an edit.

Written by the indexing model from the issue text.