white spaces
fixes the horizontal boxplot problem in #208
fixes the manual brush problem in #208
fixes the color_var() problem in #208
a single click will make the brush size 0 when the brush is in the resize mode

the reason for this change is that in the zoom mode, a single click should not zoom the plot
implement focal zoom for density plots
clean up the documentation for qdensity(); X, R, Shift are no longer used

I think we should be able to close #208 now
add di to the author of qtour()
a new function cranvas_off() to close all Qt apps, which makes it possible to run R CMD check, otherwise hundreds of plots can be opened, and they consume a lot of memory (crashed my laptop)
no need to print() plots
roxygenize
view is the second argument in qsave()
bump version; almost passes R CMD check now; the examples in time_qdata() still have problems @chxy