exec command: Add support for window stack references (%1, %@, etc)

[jordansissel]

Window references will be replaced on the command-line in the same position that they occurred. In case of %@ (all windows), multiple arguments are inserted.

For example, if the window list is two entries: 123, 456

exec echo Hello %2 will run the command exactly as this list
[echo, "Hello", "456"]

exec echo World %@ will run the command exactly as this list
[echo, "World", "123", "456"]
with each window id being a separate argument.

For #431

[jordansissel]

There's some possible area for conflict like if someone has an old command that does 'xdotool ... exec some command %1' and the user expected a literal "%1" to get passed as it has previously... but now xdotool will replace the %1 with a window id (or an error message).

Other things needing improvement before merging:

• Things like xdotool exec echo %1 should return an error code because there were no windows on the stack. At present, it only prints a message about this and still exits zero(success).
• Any documentation changes