Improve status reporting

There are some user complaints that Imager hangs at "writing 0%"
Mention in the progress status what it is actually doing prior
to writing (e.g. opening storage, blkdiscarding,
zero'ing out MBR, starting download) to help diagnose
potential issues.

main.qml

@@ -749,7 +749,7 @@ ApplicationWindow {
             cancelwritebutton.enabled = true
             cancelwritebutton.visible = true
             cancelverifybutton.enabled = true
-            progressText.text = qsTr("Writing... %1%").arg("0")
+            progressText.text = qsTr("Preparing to write...");
             progressText.visible = true
             progressBar.visible = true
             progressBar.indeterminate = true
@@ -831,6 +831,10 @@ ApplicationWindow {
         }
     }
 
+    function onPreparationStatusUpdate(msg) {
+        progressText.text = qsTr("Preparing to write... (%1)").arg(msg)
+    }
+
     function resetWriteButton() {
         progressText.visible = false
         progressBar.visible = false