feat: close window instead of hiding

[friday]

Closes #1151

After refactoring elsewhere, making the Window class much more isolated (#1378) we can now easily do this.

Two advantages:

• Instead of reusing the previous window we re-draw the window when you open it now, each time. This way for example if you search for an app, close ulauncher, and uninstall it, it will not show up next time you re-open the launcher (this was always a bug imo, but no one complained about it).
• Support user option _not_ to keep Ulauncher running in the background #1304 will now be possible and easy (although systemd integration might still cause problems for it)
• Should use less memory, but this seems insignificant when compared to Don't run extensions in the background #1063.