Device settings handling

[ixtrader]

Chnges:

1. Save "Devices" settings only when "Save" button is pressed.
2. Support a "Cancel" and "Clear" button while configuring Devices/Task table.
3. Reduce the http reply size to minimize the risk of heap overflow during device configuration
   (this problem was hurting me seriously with the board manager 1.6, the new core 2.0.0 freed at least
   15000 Bytes of dynamic memory)
4. Detect Heap overflow while configering the devices. (In case of overflow -> reboot)
5. Method WebServer handle_devices() modularized in shorter methods

prerequisite: Board manager esp8266 at least 2.0.0

[ghost]

I've done a quick test on these changes, but pulling it as it is would introduce a new bug. When the edit screen is displayed and certain dropdown menu entries are changed, data is posted back and the screen is refreshed because these changes have an effect on possible choices for other dropdown menus.

With the proposed changes this does not work anymore and you can't change these dropdown fields.

Probably has to do with this javascript code that is used in several places:

F("' LANGUAGE=javascript onchange="return dept_onchange(frmselect)">");

You can check the "Level Control" plugin to see how this fails right now.

[ixtrader]

Thanks for the feedback mvdbro, I will try to fix it.