Improve axis interaction area

[brenocq]

@epezent feedback in #24

For dragging/selecting/scrolling axes, I naturally want to hover the labels instead of the edge of the plot next to them. Consider making the label quad the widget, similar to ImPlot.

I highly agree the interaction with an axis should be done with a quad that covers both tick labels and axis labels, instead of edges. I initially wanted to implement it like this, but ended up implementing it with edges because it was easier 😛