Add docking for Vertical SplitPanel

[koca2000]

Until now SplitPanel component only support docking for Horizontal orientation. This PR adds support for docking with Vertical orientation.

[glebfox]

Hi @koca2000,

Thank you for your PR. Could you please take a look at my review comments?

Regards,
Gleb

[koca2000]

Hi @glebfox!

I made changes you suggested. I also renamed CubaHorizontalSplitPanelServerRpc to CubaDockableSplitPanelServerRpc as it more corresponds to it's usage.

Best regards,
koca2000

[glebfox]

Hi @koca2000, the changes are correct. The last thing that I found is that you need to add available dock modes to the XSD definition of split. They are located in the layout.xsd file in <xs:simpleType name="splitPanelDockMode"> element. Currently, it contains only: LEFT and RIGHT.

<xs:simpleType name="splitPanelDockMode">
    <xs:restriction base="xs:string">
        <xs:enumeration value="LEFT"/>
        <xs:enumeration value="RIGHT"/>
    </xs:restriction>
</xs:simpleType>

[koca2000]

Hi @glebfox, I added the values to XSD definition.