Allow new versions of the shell extension to be installed over the one bundled with the Krita installer

[alvinhochun]

Currently, the Krita installer force-remove the shell extension, and the shell extension won't install if Krita is installed.

The ultimate idea is to allow the user to install a newer version of the shell extension over an existing Krita installation, so to allow the user to update the shell extension without updating Krita (which is, btw, not exactly the intended use.)

If a new version of the shell extension is installed, then the Krita uninstaller would need to properly uninstall the new version. Three ways to deal with this:

• Embed the shell extension installer within the Krita installer. The shell extension installer has to accept path to krita.exe from the command line. When uninstalling Krita, just execute the shell extension uninstaller too.
  • It just seems weird?...
• Save install log and use it in the uninstaller, the shell extension installers can change the install log or place a new install log, so that the uninstaller can use them to remove the new version of the shell extension properly
  • Probably the cleanest way, but how should I generate the install log?
• In the Krita uninstaller, look for the uninstaller for the shell extension, and use it to uninstall the shell extension instead of using the included functions.
  • Probably the easiest way.

[alvinhochun]

Opted for the easiest way... Turned out to be not so easy.