tkdiff Code

forgot to bump release to 5.7

chglog updated for release 5.7

Inline instantiation realigned: prevents Heuristic+1 dropouts; broken syntax; better reading

Bookmark annotation prompt now tracks instance AFTER 1st use

Add missing trigger event when recursive inlines are activated

Miscellaneous adjusments:
- Comment out multi-monitor clipping debug msg; only serves to document TK inadequacy
- reword commentary to more accurately reflect recent changes

Remove superfluos checks for tk_version 8.5

For MacOS, don't override the color options if Tk >=8.6, since it now handles the OS dark/light theming correctly.

Fix a trucated label in the preferences window by doing font measure

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.