New widgets for ThingsBoard IoT platform.

• chart-future-timestamps.json --> This widget allows to plot a chart with values related to future timestamps, with a fixed time window. Can be useful when you need to plot forecasts of trends.
• chart-stacked-with-separate-negative-positive.json --> This widget integrates a chart.js library widget in ThingsBoard. It allows the user to plot a stacked chart which is different from the ThingsBoard standard one since it can keep separate the negative and positive values, avoiding to sum them when stacking.
• gauge-temperature-analog.json --> Analog gauge for temperature (or other type of data) which is way more configurable than the ThingsBoard standard gauges. It is an example of the integration of gauge.js library in ThingsBoard.