implement optional limit for number of tabs and LRU replacement
this allows to have the "old" behavior of pre 20.08 as option
it is not exactly the same as in e.g. 20.04, as we still use the normal QTabBar, but it should feel equivalent enough
as positive side effect, the user is no able to configure how many tabs he wants to have as maximum, default is 0, aka unlimited, before this was based on same heuristics
BUG: 426520 BUG: 426300