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