specification of qrect size in identify_rect

[mariev]

issue:

identify_rect uses magic numbers to determine range of search area [currently 1% of layer limits]

discussion:

Fitt's law generally states that the time to move to a target (in this case items that get assigned hits<-) is due to target size and distance from pointer. That is, for better UI design, use large targets.

Conversely, when the target is small, reduced transportation time can be achieved by increasing the size of the cursor.

Cranvas does not support best practices for UI design by forcing only a small cursor.

solution:

• allow cranvas plots to specify identity query rectangle via a brush attribute
• an optional argument to identify_rect [messy]
• ?? Some other, more elegant option

see also:

• https://github.com/ggobi/cranvas/blob/master/R/utils.R#L483
• http://www.sfu.ca/hmsl/Publications/Page2/chi99wang.pdf
• http://en.wikipedia.org/wiki/Fitts's_law