Rework tempo marker editing menu functions

Rework tempo marker editing menu functions

Setting a tempo to 'Continue' via right click puts it in a permanent state
of continuing the previous section's end tempo (basically what
'Lock Continue' should have been). This can be disabled (unlocked) by
selecting 'Don't Continue'.

Remove the previous temporary 'Continue' function.

Reorganise menu to separate position lock style from more commonly
used functions.