blabrowsers.py: use action constant instead of...

an integer literal to determine the button action in BlaTreeView's
button-press-event handler. Additionally, simplify color selection
in the custom CellRenderer.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Clamp alpha value of cover art between 0 and 1

This commit also moves the constants associated with the ordered
set implementation from module scope into the class definition.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Move library model class from bladb to blabrowsers

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>