[FIX] contract_variable_quantity: Allow to propagate variable qty fields from template

**Steps to reproduce the problem**

- Define a contract template with one line with variable quantity and formula.
- Use that template in a new contract.
- Variable quantity type and formula are not transferred from template to contract.

This is due to the lack of that fields in the tree view that prevents web client to fetch
and store them.

We solve it adding those fields to the contract line tree view, also providing some
visibility logic for not showing the corresponding fields according the type.

No regression tests is provided, as this is only reproducible with web client, not through
code.

contract_variable_quantity/views/contract_line_views.xml

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright 2019 Tecnativa - Pedro M. Baeza
+     License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl). -->
+<odoo>
+
+    <record id="contract_line_tree_view" model="ir.ui.view">
+        <field name="name">contract.line tree view (in contract) - Add variable qty fields</field>
+        <field name="model">contract.line</field>
+        <field name="inherit_id" ref="contract.contract_line_tree_view"/>
+        <field name="arch" type="xml">
+            <field name="quantity" position="before">
+                <field name="qty_type"/>
+                <field
+                    name="qty_formula_id"
+                    attrs="{'invisible': [('qty_type', '!=', 'variable')]}"
+                />
+            </field>
+            <field name="quantity" position="attributes">
+                <attribute name="attrs">{'invisible': [('qty_type', '!=', 'fixed')]}</attribute>
+            </field>
+        </field>
+    </record>
+
+</odoo>