Nit - replace computing constant with that constant

parsettf.c :: readdate() was computing the time difference between
the 1904 epoch time ala Mac and the 1970 epoch time ala Unix.
However this value is a constant and need not be recomputed each time.
One could inspect the computed value once and make static, or derive
the value from various references on the web.

This change replaces code and loop with initialization of a variable
with the constant value.

Tested against three different ttf/ttc files and verified from the
Element / Font Info... / Dates displays before and after change.
Fix crash #1671 gdraw/gresedit.c

GResEditDlg() has sections to set up the different kinds of controls as
found in various panes in the X resource dialog.  In two panes clicking
on the 'inherit' checkbox for the "Font:" item would crash FFC ("Text"
and "Button" panes).

GRE_InheritFontChange() runs to react to those user clicks.  It calls
GRE_FigureInheritance() to figure out the inheritance issues (whether
to inherit from the parent GGadget type).

A parameter to GRE_FigureInheritance() 'is_font' is used to flag that a
more complicated font data item is being manipulated. That flag was not
set on this call, even though a font item *was* being manipulated, which
then caused a crash in a deeper call to inherit_font_change().

Tested and cures crash.  Inheritance appears to work correctly...

During investigation, noted that descriptive title in the "Progress"
pane was wrong.  Changed this to "Progress Bars".
Amplify comments on epoch constants/computations

Better describe the derivation of the constant value now used instead
of a computation loop.  Also mention that since portable 64-bit math
was not known, the remaining loop works on 16-bit slices of the full
64-bit unsigned timestamp value.
Fix crash on GPOS kern classes select class text field

fontforgeexe/kernclass.c :: KCD_TextSelect() had several problems handling
text strings found in the "Select Class Containing:" incremental match
text field.  One could carefully tab into the field, but mouse clicking
into the field would instantly crash.

Code initially scanning string was missing check for end-of-string, which
was a problem when the field was empty.  Code was not checking the return
from u2utf8_copyn() which could return NULL on various errors.  The two
of these flaws caused the crash.

Additionally further on there was a duplicated check for end-of-string
and then a missing check that could walk off the end of the string.

Tested as fixed against both such text fields and with various input and
match strings.
Merge pull request #1678 from tshinnic/crash_kernclass_selectclass

Fix crash on GPOS kern classes select class text field
Merge pull request #1676 from tshinnic/fix_crash_1671_other_resedit

Fix crash #1671 gdraw/gresedit.c
Fix spline overlap problems by disabling the reversal removal routine.
issue1631: disable some features when in collab mode.
Merge pull request #1679 from fontforge/rmo_triangle_fix_1

Fix spline overlap problems by disabling the reversal removal routine.
Merge pull request #1681 from fontforge/monkeyiq/2014/aug/1631

issue1631: disable some features when in collab mode.
ufo: have locale-independant output
Merge pull request #1682 from adrientetar/ufo-locl

ufo: have locale-independant output
Merge pull request #1674 from tshinnic/nit_parsettf_readdate_simplify

Nit - replace computing constant with that constant