[kcms] Port simple OverlaySheet headers to use title: property

This was added to Kirigami in
https://invent.kde.org/frameworks/kirigami/-/merge_requests/314
and simplifies and standardizes the creation of simple text-based
headers in OverlaySheet. This commit ports plasma-desktop's KCMs to
use them in all the places where it is possible to do so.

kcms/users/package/contents/ui/ChangePassword.qml

@@ -23,9 +23,7 @@ Kirigami.OverlaySheet {
 
     property var account
 
-    header: Kirigami.Heading {
-        text: i18n("Change Password")
-    }
+    title: i18n("Change Password")
 
     ColumnLayout {
         id: mainColumn

kcms/users/package/contents/ui/ChangeWalletPassword.qml

@@ -14,9 +14,7 @@ import org.kde.kirigami 2.8 as Kirigami
 Kirigami.OverlaySheet {
     id: walletPasswordRoot
 
-    header: Kirigami.Heading {
-        text: i18n("Change Wallet Password?")
-    }
+    title: i18n("Change Wallet Password?")
 
     ColumnLayout {
         id: baseLayout

kcms/users/package/contents/ui/UserDetailsPage.qml

@@ -212,11 +212,9 @@ SimpleKCM {
 
     Kirigami.OverlaySheet {
         id: picturesSheet
-        header: RowLayout {
-            Kirigami.Heading {
-                text: i18n("Change Avatar")
-            }
-        }
+
+        title: i18n("Change Avatar")
+
         QQC2.SwipeView {
             id: stackSwitcher
             interactive: false