Adds two functions in ESPUI.h:
setPanelStyle()
setElementStyle()
These allow for custom inline CSS styles to be applied to the panel and to
the specific UI element repectively.
For example:
```
char stylecol1[30]
sprintf(stylecol1, "background-color: #%06X;", (unsigned int) random(0x0, 0xFFFFFF));
ESPUI.setPanelStyle(switch1, stylecol1);
```
This will set the panel of the given control to a random hex colour.
This is supported by both the initial UI message, and by control update messages, so you
can change these styles dynamically in response to other events.
setElementStyle() is not perfect. Because CSS inline styles can only style one specific
DOM element, for controls made up of multiple elements (like the "pad") this is limited.
I have tried to make an appropriate choice for each supported control.
* new member of Control: parentControl
* changed addControl() to add children of elements
* added some styles in the css
* two new divs in index.html
* added frontend code to controls.js
* added new demo to demonstrate the tabbed view
