programmatically differenciate between click and doubleclick

this is a Qt anti-feature
make the longer than a double click computation only once
remove the edge case where m_last_click could be invalid

the class now track the next valid click time instead of the last click time
making the logic of determining if we are in a double click a little simpler
since we can initialize m_end_of_double_click to a valid value when instantiating
the class.
Merge branch '5229-switchboard-double-click'