Add rendering and editing support for gradients

[alexjlockwood]

[Shusshu]

Can you support importing gradient (even without visualisation and editing) so it can be exported to vector drawable?

<aapt:attr name="android:fillColor">
        <gradient
                android:type="linear"
                android:angle="45"
                android:startColor="#00FFFFFF"
                android:endColor="#FFFFFFFF"/>
    </aapt:attr>