Windows MSI/WiX: Rework, Bugfixes

• Update to WiX v4
• (Implicitly) change InstallScope to perMachine (CMake does this automatically when using WiX v4)
  • Fixes #1617 (closed) #1618 (closed)
• Abort with error if a previous (MSI- or EXE-) installation is found. This means that users are forced to uninstall the old version first. This is not nice but currently necessary to avoids broken installations. We can soften the check in the future, once we have found out how to produce .MSI that cleanly update from one version to the next. Currently, even upgrading from 1.3.0 MSI to 1.3.2 MSI results in a broken installation.
  • Fixes #5126 (closed)
  • Related: #4679 (closed)

Tests:

If not otherwise mentioned, the MSI was opened by double-clicking in Explorer.

• 1.3.2 EXE --> 1.5-dev-patched MSI (expected: refuses to install)
• 1.3.2 MSI --> 1.5-dev-patched MSI (expected: refuses to install)
• 1.3.2 MSI --> 1.5-dev-patched MSI, via msiexec commandline (expected: refuses to install)
• 1.4 MSI -> 1.5-dev-patched MSI (expected: refuses to install)
• nothing --> 1.5-dev-patched MSI (expected: correctly installs)
• 1.5-dev-patched MSI -> nothing (uninstall via system settings)
• 1.5-dev-patched MSI -> 1.5-dev-patched MSI (opens "Change/Repair/Remove menu")
  • Change-installation
  • Repair-installation
  • Remove-installation