switch to syncthingtray

syncthing-gtk appears not to be being maintained anymore, and doesn't
support Python 3, while a dependency of it no longer supports Python
2, so it can't easily be built in nixpkgs.

See kozec/syncthing-gtk#487

.config/nixpkgs/overlays/profile.nix

@@ -17,7 +17,6 @@ self: super: {
       numix-icon-theme
       numix-icon-theme-square   # application icons
       psmisc
-      syncthing-gtk
       source-code-pro
       tree
       xmonad-custom

.config/nixpkgs/overlays/xmonad/default.nix

@@ -15,7 +15,7 @@ self: super: {
           { pkg = self.networkmanagerapplet; path = "/bin/nm-applet"; args = "--indicator"; }
           { pkg = self.system-config-printer; path = "/bin/system-config-printer-applet"; }
           { pkg = self.udiskie; path = "/bin/udiskie"; args = "--tray --appindicator"; }
-          { pkg = self.syncthing-gtk; path = "/bin/syncthing-gtk"; args = "--minimized"; }
+          { pkg = self.syncthingtray; path = "/bin/syncthingtray"; }
           { pkg = self.lightlocker; path = "/bin/light-locker"; args = "--lock-on-suspend --late-locking"; }
           { pkg = self.xmonad-custom; path = "/bin/launch-taffybar"; logName = "taffybar"; }
           { pkg = self.keepassxc; path = "/bin/keepassxc"; }