config: i3: replace urxvt with alacritty

Seems to be faster and triggers less graphical glitches in conjunction
with gdb (examine).

config/i3/config

@@ -24,7 +24,7 @@ floating_modifier $mod
 
 # start a terminal
 #bindsym $mod+Return exec i3-sensible-terminal
-bindsym $mod+Return exec urxvt
+bindsym $mod+Return exec alacritty
 
 bindsym $mod+b exec chromium