feat: support transparency (RGBA) in i3bar (#3727)

We introduce a --transparency flag for i3bar in order to enable a mode which
supports the use of RGBA colors.

An important constraint here is that tray icons will always have
a fully transparent background.

fixes #3723

i3bar/include/configuration.h

@@ -48,6 +48,7 @@ typedef struct config_t {
 
     position_t position;
     bool verbose;
+    bool transparency;
     struct xcb_color_strings_t colors;
     bool disable_binding_mode_indicator;
     bool disable_ws;