Add inactivity timer to switch screen off

[tomsci]

Also:

• Added "inactivity_time" (30s) and "boot_nosleep_time" (15s) settings
• Button presses automatically reset screen inactivity timer
• Fix TextWindow redraw if number of lines in title changes
• Apps using Wi-Fi or USB no longer call set_sleep_enabled(False), this is handled automatically based on whether Wi-Fi interface is up or if USB has enumerated.
• Remove Web REPL from main menu as it's no longer useful
• Fixed broken cancellation of on_up_down() buttons in deactivate

[tomsci]

@MatthewWilkes any comments on this? I'm pretty happy with how it's behaving in that plugging in USB seems to do the right thing regardless of what state the board is in.

regarding LCD switchoff, the way I've implemented it, if we're not lightsleeping because of USB or wifi on, the LCD stays on all the time. This should be a clue to the user that they need to do something (eg reset the board) to get sleep working again. It seemed simpler to do this than to allow lightsleep and lcd off to be orthogonal.

It'd be nice if usb_connected() could reset once USB is unplugged again, and if we could register some sort of callback into micropython when that happens, but those are nice-to-haves :)

[MatthewWilkes]

The connected flag is supposed to reset, but I could look at using mounted instead.