CustomForms - support subForms

[rsoika]

Support a new optional tag to separate imixs-form-sections into tabs

<?xml version="1.0"?>
<imixs-form>

    <imixs-subform label="Basic"  >
        <imixs-form-section columns="1" label="Formular">
            <item name="debug.name" type="text"  label="Name:" />
        </imixs-form-section>
        <imixs-form-section columns="2">
            <item name="debug.user" type="custom" path="userinput"  label="Responsible2:" />
            <item name="debug.team" type="custom" path="userlistinput"  label="Team:" />
            <item name="debug.country" type="text"  label="Country:" span="4" />
        </imixs-form-section>
    </imixs-subform>

    <imixs-subform label="Dispatch" readonly="false">
        <imixs-form-section label="" path="sub_dispatch" />
    </imixs-subform>

</imixs-form>

This feature should be optional and not break existing form-definitions!