nemo-desktop: Don't crash/quit in Wayland when the monitor is removed (…

…linuxmint#3785)

Hold the application alive in Wayland mode when monitors disappear,
so it can recover when the monitor returns, rather than exiting.

Handle GdkDisplay::monitor-removed immediately, by destroying windows
cleanly before gtk-layer-shell's closed handler fires, so there's no
dangling surface for the frame clock to hit.

nemo-view.c: fix crash after some shortcuts (linuxmint#3525)

On action_follow_symlink_callback() and
action_open_containing_folder_callback() we now confirm that exactly one
thing is selected.  Previously, if zero things were selected, we would
segfault.

nemo-view.c: fix crash after some shortcuts (linuxmint#3525)

On action_follow_symlink_callback() and
action_open_containing_folder_callback() we now confirm that exactly one
thing is selected.  Previously, if zero things were selected, we would
segfault.