dock:: read theme icon from /etc/deepin-version

tower: https://tower.im/projects/8f19f0bf0e754f0b82ef2c24bc230973/todos/72ae707bcf0d451b9d80e2350189f58d/#2c783ad512dc4ad7bb4ff8f34131624d

Change-Id: Ib21c3e0836306fa77aa4ecca4f341d79dc82a574