Adjust menu field's menu bar height in auto mode

On IRC diver pointed out to me that KeymapSwitcher had a menu field that
was drawing as just a line since my recent change to BMenuField. I did a
little research and discovered that this was because the menu field in
KeymapSwitch was not using the layout APIs and it's frame rect was set to
0 height.

I did a little more research and experimented with menu fields in
BeOS R5. I discovered that in R5 if the menu field is set to auto-size
mode then the menu bar inside ignores the height of the menu field frame
and uses the BMenuBar's preferred height instead.

So, I adjusted the BMenuField code in Haiku accordingly. This should make
Haiku match the behavior of BeOS R5 in auto-size mode. For fixed-size mode
it should also work the same, although some more testing is needed to
see if there are any regressions there.
Remove NotifyNodesCleared() again.

- For various reasons this one can be error prone, since it relies on
the model being able to provide the correct row count, which won't be
the case if the subclass calls it after having already removed all its
nodes.

- Optimize Table's RowsRemoved() similarly to TreeTable's for the remove
all rows case.
VariablesView: Switch back to NotifyNodesRemoved().
ImageFunctionsView: Switch back to NotifyNodesRemoved().