Adding level and slope trigger

Trigger is an important feature of the oscilloscopes. The feature has been suggested and requested by users as part of the feedback.

It's possible to implement the triggers in two ways:
- **Hardware trigger**: This method would require the ADC chips to include a digitally programmable comparator (e.g. ADS1015). This would not work for all ADCs.
- **Software trigger**: In this method, one can simply compare the measured value with the trigger value and thus plot the required signal. This is easy to implement and would work irrespective of the type of an ADC.

Since the support for more ADCs are planned, it would be better to go with software triggers. Any suggestions are welcome. Thanks!