various fixes, improvements and new features:
polish and stabilize the 'preset' feature
fix bug in gfilter(..., initial.vars=)
make gfilter(..., initial.vars=) more flexible to allow the new 'preset' filter
always select 1st item in 'variable' filter dialogue
allow user to temporarily disable filter (as opposed to permanently remove filter)
disable 'include NA' for 'preset' filter
by default do NOT select any items in 'multiple' filter (seems more consistent to me, and more useful for the user)
rename 'include NA' label to 'NA' and add a more descriptive tooltip
add seq_sane() fun to allow defining of "sane" sequence with "intelligent" steps
default the 'preset' filter to min(nrow(DF), 1000) lines
in 'preset' switch from gslider() to gcombobox() for increased flexibility in defining the steps (see seq_sane)