gtk: add new-window action to .desktop files

Inspired by #413.

This modifies the .desktop files so that it's possible to right-click on
the Ghostty icon in a dock and get the "New window" action.

Apparently this is how you implement that in GTK applications. In order
for us to also get "New tab", we'd have to implement a CLI flag, I
think.

dist/linux/app-flatpak.desktop

@@ -7,3 +7,8 @@ Icon=com.mitchellh.ghostty
 Keywords=terminal;tty;pty;
 StartupNotify=true
 Terminal=false
+Actions=new-window
+
+[Desktop Action new-window]
+Actions=new-window
+Exec=/app/bin/ghostty

dist/linux/app.desktop

@@ -7,3 +7,8 @@ Icon=com.mitchellh.ghostty
 Keywords=terminal;tty;pty;
 StartupNotify=true
 Terminal=false
+Actions=new-window
+
+[Desktop Action new-window]
+Actions=new-window
+Exec=/usr/bin/ghostty