tkdiff Code

Miscellaneous fixes:
- Popup menu activation permitted when over L/R scrollbars
- Somewhat useless Dbg message for FAILED edge-clipping .vs. multi-monitor conditions
Basically documents intractability of problem - inadequate TK information available.
- Initialization to avoid situational 'missing variable' error for highly unlikely
mouse position during startup position when positioned over a Tooltip location.

Adjust math to attempt CENTERING Thumb of the DiffMap pseudo-scrollbar relative to the colorbars.
Sadly still imperfect (generally) due to mathematically induced vertical shift from ratio of linecount
to available pixel height, combined with an enforced minimum Thumb height.

Allow implied "null content" (eg. UNIX "/dev/null" or Windows "nul") files to Diff, despite the obvious result.

V5.6
* Added recursion ability into directory searching for the 1-FSpec case: allows trees
* Recursion (both single and dual FSpec forms) supports patterned-fname exclusions
* Repaired merge-preview window jump-scroll action when current hunk is changed.
* Handful of spelling typos, coding consolidations, and minor "Help" clarifications.

Repair typo-caused crash PLUS ineffective prior VPATH-logic fix; also clarify help-info

better recognize EFFECTIVELY empty CustomCode

Repair missing-Var crashes plus minor initialization faults

Update CHANGELOG for V5.5.1

Minor logic errors in Pref-morphing repaired for fast-track V5.5.1 release

Repair spurious typos, mangled wrap protection and new-user prefs logic faults