Add convenient mouse commands for status bar

Thunar for easy lazy access to files (eg. videos), and gxkill which is a
tiny wrapper around xkill -- to "close" any window.

dotfiles/i3/config

 # finds out, if available)
 bar {
         status_command i3status
+        bindsym button2 exec thunar
+        bindsym button3 exec gxkill
         separator_symbol "•"
         # other suggestions:
         #     ‧•┇╻╹╹╿═╎╏◆◾║︙⎪∣∕⟘⟙∙∼⦀⦑⦒⦚⧸⧹▓▒░⧻⧺┆˘͡͡ːˌ˷｜･፧።