Ensure that such use cases as forced-csd environments don't cause the desktop window to be decorated

* Do not attempt to unref or destroy a child of a container

*In GTK 3 at least, GtkContainers automatically destroy child widgets when destroyed

As done for copy/delete jobs, restart the operation timer also for move
jobs. Thus preparations (e.g. `scan_sources`) later won't affect the
later transfer rate calculation.

caja-file-operations: stop timer when waiting

When a copy/move operation is created while another operation is already
active, the new operation is queued. As the (already running) operation
timer of the new operation is not stopped during the waiting period,
that period is (erroneously) included in the transfer rate calculation
and leads to initially low/slowly increasing transfer rates be shown.

Hence stop the operation timer when the (queued) operation is waiting.

Fixes #1420 and #1623.

* caja-file-operations: fix estimate for queued copy

Fixes the condition for showing an estimate of the remaining duration in
case a copy operation is queued, correctly considering the current
transfer rate.

* caja-file-operations: fix division by 0 for delete

Aligning to the copy operation case, this fixes the condition for
showing an estimate of the remaining duration for delete operations,
preventing a possible division by 0 due to a zero transfer rate.

*Do not use G_OBJECT_CLASS (class)->constructed
*We don't seem to need it anymore and it breaks dragging windows with the titlebar or resizing with the mouse in wayland

For certain operations, the destination is checked. If an error occurs,
the user is asked how to proceed using a dialogue. So far this dialogue
was hard-coded to only consider the copy operation.

This fix also considers the other affected operations and generalizes
the continue button text.