qt: Rename 'Confirmed' balance to 'Spendable' on overview page

The word 'Spendable' more precisely says what the balance actually means.

Avoids the confirmed/unconfirmed confusion that can be caused by #3676.

src/qt/forms/overviewpage.ui

@@ -105,7 +105,7 @@
             <item row="0" column="0">
              <widget class="QLabel" name="label">
               <property name="text">
-               <string>Confirmed:</string>
+               <string>Available:</string>
               </property>
              </widget>
             </item>
@@ -137,7 +137,7 @@
             <item row="1" column="0">
              <widget class="QLabel" name="label_3">
               <property name="text">
-               <string>Unconfirmed:</string>
+               <string>Pending:</string>
               </property>
              </widget>
             </item>