Add bar_inactive_color property for ScrollView

Lets the user set the color and opacity of the scroll bar, when no scroll is happening.
In addition, the same values will be used, when scroll_type = ['content'].
update ScrollView kv definition
pep8 fix
bind bar_color and bar_inactive_color to their respective states
fix method name
fix binded method name
unschedule bar color change before unbinding
Merge pull request #2296 from dessant/patch-12

Add bar_inactive_color property for ScrollView