Closes #607

The PR adds a ImPlotAxisFlags_TickLabelsInside flag that moves the tick labels into the plot area, saving space around the plot and making the plot area larger. It applies only to the innermost axes - at most 4 a time (X + Y and default + opposite).

It's useful when multiple plots have to be aligned, but some of them have much longer labels than the others and the final padding becomes significant.