add multiple slots in gesture detector
So that gesture detecting in one slot won't block gestures in the other slots.
cleanup: use local variables in time arithmetic
add gesture emitting rate in GestureRange
set rate in pan GestureRange in bbox widget
add direction and distance in pan/swipe gesture
add flipping mode in pdf/djvu reader
By tapping on the upper left corner of the reader one can toggling flipping mode on/off.
In flipping mode swiping right/down and left/up  will paging backward and forward
accordingly with pages proportional to swiping distance.
add resources files for page flipping
Merge pull request #776 from chrox/mt_hold

add multiple slots in gesture detector