Second large-scale refactoring (too large, bad habit):
All Actions' effect delegated to their respective class.
Interactions to get the page renamed to backtrace and added back to Page class
Timer permanently turned on and results are written into backtrace (instead of pageValues)
pageValues (cache of elements info) removed, will enable later
pre-populated pageValues renamed to context
PageWithValues removed
Add ActionChain to hold context & seq[Actions], add a few operators for quick modification
Clean up useless codes