Rename function `nemo_window_pane_close_slot'

The header for nemo-window-pane.h exposes the functions
`nemo_window_pane_close_slot' as well as `nemo_window_pane_slot_close'.
This naming is just unfortunate. To try and clarify the situation a bit,
rename `nemo_window_pane_close_slot' to
`nemo_window_pane_remove_slot_unsafe' as the latter really just removes
the slot from the GtkNotebook. The `*_unsafe' portion of the name refers
to the fact that it is not safe to call the function with the active
slot of a pane as this might crash Nemo.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Rename function `nemo_window_pane_slot_close' to..

`nemo_window_pane_close_slot'.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Switch to next tab after closing the active one

Closing the active tab would previously select the first inactive slot
in a pane. It is more common in other applications though to select
either the next slot (if one exists) or the previous one. Note that we
have to consult the GtkNotebook which holds the slots for this instead
of the pane's list of slots since the latter does not necessarily
represent the order in which slots are actually displayed in Nemo.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Merge pull request #651 from nkoep/rework-tab-closing

Rework tab closing