Fix vertical Slider

Slider, when vertical, was drawing the grab box with inverted position. Center "track" had incorrect orientation.

Also, slider.lua was passing a bunk table to group.getRect().
Tweak Slider style

Made a marginally simpler & more readable Slider draw routine using rectangles instead of lines.
Also the ends of the slider "track" are capped now. I split this commit in case a rollback is desired.
Only show Input cursor when selected

Previously all Inputs would show the text cursor, which looks odd and makes
it harder for the user to see which one is selected.