'Text Entry...' dialog should respect 'text-size' option's value

[dmn]

1) A size value for new text as it appears in
'Text Entry...' dialog is a 'magic number' (12)
hardcoded in gschem/src/x_newtext.c:277.
Apparently, user-defined 'text-size' value from
.gschemrc or system default from system-gschemrc
should be used instead.

2) g_rc_text_size() function in gschem/src/g_rc.c
also uses hardcoded values for minimum and default
text size. But there are MINIMUM_TEXT_SIZE and
DEFAULT_TEXT_SIZE constants defined in
libgeda/include/libgeda/geda_text_object.h
that should be used.