This commit changes how commands decide what container to act on.
Commands get the current container though `current_container`, a global
defined in sway/commands.c. If a criteria is given before a command,
then the following command will be run once for every container the
criteria matches with a reference to the matching container in
'current_container'. Commands should use this instead of
`get_focused_container()` from now on.
This commit also fixes a few (minor) mistakes made in implementing marks
such as non-escaped arrows in sway(5) and calling the "mark" command
"floating" by accident. It also cleans up `criteria.c` in a few places.
