tasks_analysis: Add a multiple-tasks CPUs residency plot

This provides a new convenient task analysis API to generate a stacked
plot of CPUs residency time for a list of specified tasks.

Such an analysis could be useful to identify where a bunch of important
tasks (e.g. top-app tasks) spend most of their time. The generated
dataframe and plots sorts tasks by increasing running time, thus making
it even more simple to identify the most active tasks.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>