Make the sidebar wider

Ratio 'sidebar's width'/'button's height' in the mockup:
300px/32px

Button's height on Fedora 21 and gtk3-3.13.7-1.fc21.x86_64:
33px

The correct width:
(33*(300/32)) = 309.375px ~= 310px

Related to #83

Signed-off-by: Jakub Filak <jfilak@redhat.com>