e-window: use a css file for widget styling

Replace local styling (used for the tab close button and embed's status
frame) by a global css sheet loaded at window construction.
This allows tweaking style properties without rebuilding.

The css file is installed in $(pkgdatadir)/epiphany.css.

Signed-off-by: Diego Escalante Urrelo <descalante@igalia.com>

Bug #644805