fix: timer callback being executed after timer:stop

[jimmyl0l3c]

Fixes #27, fixes #19 , fixes #23.

How to reproduce the bug I am fixing:

• Call ShowkeysToggle
• Press a random character, for example h
• While the showkeys window is visible, press some operation that waits for motion, like d, this takes you to operator-pending mode
• When timer of the pending operation (d in this example) fires the utils.clear_and_close callback, the error occurs

The cause: the timer of the character pressed before entering operator-pending mode (h in my example) gets scheduled before it gets cancelled by timer:stop when handling the pending operation (d in my example). Because of this, the callback gets executed twice and during the second execution, the state.win is nil, causing the error.

My solution is to save timer_id, so we can check whether the current callback is the last scheduled one.