[Companion] SD Sync reworked: (#4906)

* Complete re-write of SyncProcess, now more robust, UI-independent, runs in own thread, large speed boost;
    * Use file hash comparisons instead of byte-level compare with string methods (probable fixes for #4837 and #4848);
    * New pre-sync options dialog with selectable folders and other settings (settings are remembered until app exit);
    * Option to choose sync direction(s) (up/down/bi) and the order in which bi-directional sync runs;
    * Option to choose file comparison mode (timestamp/hash/both) or force overwrite (for one-way sync);
    * Option to set a maximum file size to copy;
    * Option to do a test run w/out actually copying any files;
    * Better reporting of progress, total counts (copied/updated/skipped), and errors, with color coding;
    * Much more reliable to cancel long-running/hung copy process;
    * ProgressWidget can now display rich-text in the text browser, added some new functions for usage;
    * Misc. small layout tweaks to ProgressDialog and ProgressWidget to make them behave/look better;

companion/src/progressdialog.cpp

@@ -32,6 +32,7 @@ locked(false)
   ui->setupUi(this);
   setWindowTitle(title);
   setWindowIcon(icon);
+  setWindowFlags(windowFlags() & ~Qt::WindowContextHelpButtonHint);
   if (forceOpen) {
     ui->outputProgress->forceOpen();
   }
@@ -53,6 +54,7 @@ void ProgressDialog::on_closeButton_clicked()
 {
   if (!locked) {
     ui->outputProgress->stop();
+    emit rejected();
     close();
   }
 }
@@ -77,3 +79,13 @@ bool ProgressDialog::isEmpty() const
 {
   return ui->outputProgress->isEmpty();
 }
+
+void ProgressDialog::setProcessStarted()
+{
+  ui->closeButton->setText(tr("Cancel"));
+}
+
+void ProgressDialog::setProcessStopped()
+{
+  ui->closeButton->setText(tr("Close"));
+}