Provide instructions to Windows users about uninstalling previous Dangerzone versions

[apyrgio]

As a result of #929, Windows users who have install Dangerzone before will be greeted with a message that says they need to uninstall previous versions, in order to install the new one. In order to help these users, here's what we can do:

• In our website, keep the download button for the Windows MSI as is.

• If users have not installed Dangerzone before, they will not be affected.

• Write a descriptive error message, for the users who have Dangerzone installed. Something like:

  We detected that a previous Dangerzone version is installed in your system. Dangerzone versions prior to 0.9.0 must be uninstalled from "Programs and Features", before installing a new version. Read more about this change in:

• Add a section in our dangerzone.rocks website or our GitHub wiki page where we can explain with screenshots how to uninstall a previous Dangerzone version, and install the new one. We can also link to this PR for more context.

Hopefully this will be a smooth transition as any.

Originally posted by @apyrgio in #929 (comment)

[apyrgio]

I have experimented a bit on this front, and I have some more info about the dialog box that we can show to the user:

1. That dialog box cannot have rich content, e.g., URLs. That is, without specifying custom elements with Wix, which I'd prefer not doing.
2. There's a length restriction for the dialog box. Strings past a certain size get truncated.
3. The text in the box cannot be copied.

Currently, this is how it looks like:

A more descriptive error message could be:

But I'm wondering how we can point users to our website from such an error message. If we add a URL there, it will have to be short (because it must be typed), and we may need to remove our uninstallation instructions. If we don't add a URL, then we lose a link to a content with pictures and rationale. Not sure what's best...