adwaita: add gradient and shadow to active tasklist box

In current Adwaita theme, the gradients and box shadows give the tasklist button a slight 3D look. But currently active buttons do not have this look and appear flat because we use a solid background color instead of a gradient and also because we remove the box-shadow.

This patch adds: linear gradient, box shadow, bottom border and text shadow to active tasklist buttons to make the appearance of the active button more consistent with the appearance of default and hover styles of the tasklist buttons. Active buttons will now appear darker than the other buttons but otherwise have the same 3D look than other buttons. I found these values by trial and error, trying different values until I found a value what I found looked good.

Here is an example of the current Adwaita style:

Here is an example of the updated Adawaita style: