Link picker: add link form

<?xml version="1.0" encoding="UTF-8"?>

<form>
    <tab id="basic" sortorder="10" title="preside-objects.link:basic.tab.title">
        <fieldset id="standard" sortorder="10">
            <field sortorder="10" binding="link.type" control="hidden" />
            <field sortorder="20" binding="link.internal_title"        />
        </fieldset>

        <!-- we will show/hide these fieldsets depending on the selected link type -->
        <fieldset id="sitetree" sortorder="20">
            <field sortorder="10" binding="link.page"        />
            <field sortorder="15" binding="link.query_string" control="textinput">
                <rule validator="queryString" />
            </field>
            <field sortorder="20" binding="link.page_anchor" />
        </fieldset>

        <fieldset id="url" sortorder="30">
            <field sortorder="10" binding="link.external_protocol" control="select" values="http://,https://,ftp://,news://,tel://" addMissingValues="true" />
            <field sortorder="20" binding="link.external_address"  control="textinput" />
        </fieldset>

        <fieldset id="email" sortorder="40">
            <field sortorder="10" binding="link.email_address" control="textinput"   />
            <field sortorder="20" binding="link.email_subject" control="textinput"   />
            <field sortorder="30" binding="link.email_body"                          />
            <field sortorder="40" binding="link.email_anti_spam" defaultValue="true" />
        </fieldset>

        <fieldset id="asset" sortorder="50">
            <field sortorder="10" binding="link.asset"    />
            <field sortorder="20" binding="link.download" />
        </fieldset>

        <fieldset id="anchor" sortorder="60">
            <field sortorder="10" binding="link.page_anchor" />
        </fieldset>
    </tab>

    <tab id="content" sortorder="20" title="preside-objects.link:content.tab.title">
        <fieldset id="content" sortorder="10">
            <field sortorder="10" binding="link.title" control="textinput" />
            <field sortorder="20" binding="link.text"  control="textinput" />
            <field sortorder="30" binding="link.image"                     />
            <field sortorder="40" binding="link.target" control="select" values="_self,_blank,_parent,_top" labels="preside-objects.link:targets.self,preside-objects.link:targets.blank,preside-objects.link:targets.parent,preside-objects.link:targets.top" />
            <field sortorder="50" binding="link.nofollow" />
            <field sortorder="60" binding="link.referrer_policy" />
        </fieldset>
    </tab>
</form>