Windows installer - Add entry in App Paths registry key

This patch ensures that both Windows installers (exe, msi) create a registry key under

HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\App Paths\inkscape.exe

with "default value" entry for path to inkscape.exe and "path" entry for path to directory into which inkscape.exe is installed.

I successfully tested it as follows:

• Built 64- and 32-bit versions of Inkscape from upstream/master (before commit 25827d27)

• For both architectures I built the exe and msi installer

• Installed Inkscape using each of the installers in the default installation location as well as in a user defined location

• Checked for the existance of the keys from a command prompt

  reg query "HKLM\Software\Microsoft\Windows\CurrentVersion\App Paths\inkscape.exe"

  which results for example in

  HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\App Paths\inkscape.exe
  (Standard)    REG_SZ    C:\Program Files (x86)\Inkscape\bin\inkscape.exe
  Path          REG_SZ    C:\Program Files (x86)\Inkscape\bin\

• Uninstalled Inkscape and cheched if the keys are removed

So I think this is ok now. Two things remain open:

1. For the msi installer the instructions for writing in the registry is in the file packaging/wix/file_association.xml. It cannot be placed into a separate file since CPack does not allow to inject code into the same component from different fragment files. I recommend to rename the file, e.g. app_registration.xml What do you think @ede123?

2. I am not very happy to have separate entries for the installers (exe installer: ConfigCPack.cmake, msi installer: file_association.xml). So I think it is really worth to get this directly into CPack. Since this will take a lot of time I recommend to keep this workaround until we might have a more elegant CPack-solution.

Resolves #2255 (closed)