GitLab

This reverts commit dc854fb3.

The idea is to be able to build sources from already cloned git submodules
instead of getting the sources via the ExternalProjects.

prepare.py: add option -f/--force to allow regeneration even if working directory exists (disabled by default)

prepare.py: stop generation if working directory already exists to avoid conflicts between multiple executions

- Include git revision in the filename
- Add banner image in the installer
- Propose to launch linphone at the end of the install

Create a windows shortcut to linphone.exe in the install directory on Windows when building for desktop.