Windows MSI installer: make install directory configurable

[SunderB]

(Hopefully) allows the user to choose the installation directory instead of always installing in Program Files.
Closes issue #2782 and discussion #2774

I'm not familiar with WIX or how it works, but I found this: https://wixtoolset.org/documentation/manual/v3/wixui/dialog_reference/wixui_installdir.html which is what I've based this PR from.
This has not been tested yet. Testing is greatly appreciated!

[ethancrawford]

Looks great! I'll test this out in a week or so unless someone else gets to it before me 😄

[ethancrawford]

It's been a bit more than a week 😂 I've been quite busy recently... will report how I go in the next few days 👍

[ethancrawford]

So, initial tests show that it displays a dialog for picking the install directory.
However, when it finishes the install process, it appears to still install into the original program files location.
I'll see if I can troubleshoot this a little more.

[ethancrawford]

I don't know whether it's related, but when picking the directory to install to, I get this error: (which apparently comes from Windows Installer). Clicking Ok seems to still allow me to move on to the next stage of the installer.

The end result is still as in my above message.

If I do something drastic like remove the wrapping <Directory Id="ProgramFiles64Folder" Name="PFiles"> in sonic-pi.wxs, then it seems to put Sonic Pi in the chosen location, but the app crashes on starting.
I also saw an article that seemed to say that the ids of the nested directories should all not be in uppercase, but if I change them to lowercase, it makes no difference for me.