7-Zip with an enhanced SFX component

Since its inception in 2007, Git for Windows made use of what was then called the "modified SFX" that used to be available from http://7zsfx.info. Sadly, that project seems to have gone defunct some time during 2016 and nobody seems to know why.

Git for Windows used the "modified SFX" instead of the one shipped with the official 7-Zip, for the following reasons:

• The portable Git distribution wants to install into the final location rather than into a temporary directory (which would get deleted after the SFX ran the included setup.exe). The "modified SFX" supports configuration to ask the user where to extract/install the archive.
• When canceling the installation, it is nice to be able to ask the user whether they are sure, and the "modified SFX" supported that.
• The "modified SFX" allowed to set the window title of the "BeginPrompt" window as well as the progress window.

This fork supports those needs of the portable Git for Windows package. The default branch always be the home for Git for Windows' modifications, switching to a new branch when newer 7-Zip version are released, rebasing the changes from the previous default branch.

Please note that this project does not aim for a full replacement of the "modified SFX" so far:

• there is no support yet for any of the --sfxconfig, --sfxversion, --sfxlang, --sfxtest, --sfxwaitall, and the --sfxelevation command-line option.
• a lot of features are not reimplemented in this project, such as the GUIMode, the GUIFlags, or the OverwriteMode setting, just to name a few.

Pull Requests that implement interesting and useful features are warmly welcome!