Sleep Timer Fix Reload by Miguel

Based on "Tab Sleep Timer" by Daniel de Haas and bugfixes in "Tab Sleep Timer Fix" by Mitch Crane.

If you are listening to music:
- "reload" stops the music: select this if you want to use the tab again later (e.g. listen to another program)
- "close" stops the music: select this if you will not need the tab anymore
- "mute" (not available on Android) does not stop the music but you will not hear it anymore: select this if for some reason you need the web page to go on without listening to it

Note: some websites may request a confirmation to close the tab when the timer is about to “Reload” or “Close” a tab. Usually, the music will still stop shortly after, because the data flow is interrupted. To prevent this problem, the timer removes any code inserted by "onbeforeunload" before trying to reload or close a tab (should this behavior be configurable?). If the code was inserted by AddEventListener("beforeonload",...), nothing can be done to remove the confirmation.

History
v1.0: original add-on by Daniel de Haas
v1.0.5: bug fix by Mitch Crane
v1.1: largely rewritten. Ability to use the keyboard to enter the duration. On Android, first click on the duration to open the virtual keyboard.
v1.1.2: bugfixes; prevents time from going over limits
v1.1.3: display add-on icon in red when it is counting down; remove .onbeforeonload when going “Reload” or “Close”