Allow / separated pattern to filter same filename across different directories

Suppose you have the following dir structure:

abc/main.cpp
def/main.cpp

Using quickopen it is very difficult to find the main.cpp that you want. This is because we remove the filename from the path so the filter doesn't work if you try for e.g "ab/ma".

For now, I have made it so that the filename is always present in the path but is removed during display. This allows for more flexible filtering.

I have also included an optimization for the case where you have too many rows and you select something from very bottom. This was resulting in too many unnecessary "sizeHint" calculations.