plugin/base: rewrite t() function to use todo.sh

Move the test for whether `plugin/todo` is enabled inside the function, and remove the alias from there.

Alsö, respect `$XDG_STATE_HOME` and move the old `~/.t` file if it exists.

plugins/available/todo.plugin.bash

@@ -4,5 +4,3 @@ about-plugin 'Todo.txt integration'
 # you may override any of the exported variables below in your .bash_profile
 : "${TODOTXT_DEFAULT_ACTION:=ls}"
 export TODOTXT_DEFAULT_ACTION
-
-alias t='todo.sh'