rename level "editor" to less confusing "selector"

Editor is confusing wrt "data editing". Selector would be a better alternative.
rename 'clear' to more appropriate 'reset' button
do not use radio buttons for 'single' choice filter

when a variable has 3 levels or less, gfilter uses radio buttons when 
using 'single' choice selector. Although this is generally a good idea, 
it becomes less useful when the levels are very long. Unfortunately in 
this case the widget eats too much horizontal space. Using a combobox 
all the time solves the issue.
resize 'multiple' choice selection when appropriate
Merge pull request #5 from landroni/gfilter-tweaks-b

Misc gfilter tweaks
Merge pull request #2 from jverzani/master

update to upstream master