Slider fixes

[caryoscelus]

Several fixes for gradient/hue sliders:

• jump to value on click (fixes #31)
• add colorChanged signal (#30)
• fix vertical hue slider (it was inverted before)

[caryoscelus]

I guess i'll just push fixes here before they i get branches messed up.

[mbasaglia]

Just wanted to let you know that I haven't forgotten about this merge request but haven't gotten time to properly review it just yet. Hopefully I'll have some free time this Sunday.

[caryoscelus]

I created an issue for inverted appearance - i'll look into it next time i tackle sliders; /0 issue is fixed.