Windows Installer fixes
Several fixes:
- Gajim-Portable: Adds a check for an existing install, and if found, asks the user if its ok to remove that (without removing UserData). Fixes #10477 (closed) and #9025 (closed)
- Gajim: Kills gdbus.exe if it is still running after quitting Gajim. Fixes #10539 (closed)
- Replaces "Nullsoft Installer [...]" branding text with "Gajim Setup"