Guard rail for clicking refresh with refresh already in progress

[HebaruSan]

Problem

CKAN's tray icon has a notification that pops up after a refresh when there are updatable mods. Clicking this begins installation of those updates.

If you do this while an update or install is already in progress, an exception is thrown:

System.InvalidOperationException: This BackgroundWorker is currently busy and cannot run multiple tasks concurrently.
   at System.ComponentModel.BackgroundWorker.RunWorkerAsync(Object argument)
   at CKAN.GUI.Main.minimizeNotifyIcon_BalloonTipClicked(Object sender, EventArgs e)
   at System.Windows.Forms.NotifyIcon.OnBalloonTipClicked()
   at System.Windows.Forms.NotifyIcon.WndProc(Message& msg)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

Cause

Similar to #4302, when the progress screen is already busy doing something, it can't be interrupted to do a refresh instead / in addition.

Changes

• Now if the progress screen is already busy, attempting to start another refresh will do nothing, so the exception won't be thrown.
• The English text of the notification is slightly re-worded to hopefully make it easier to understand.
  

Fixes #4295.