* 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.

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.

*Draw the background on the desktop window, using the same code as on navigation windows to bypass the no-root-window issue.
*Works from caja's "Change Desktop Background" menu item, from caja-extensions "set as wallpaper" and with changing the background in dconf-editor.
*All background options available in x11 supported in wayland, UI is new for wayland

*Apply changes immediately, keep the dialog open until closed by user
*Let users see changes in realtime while keeping the dialog open
*Preview both colors and pictures
*Add ability to apply changes from either combobox including using any image saved from when dialog was opened
*Add tooltips to the buttons, filepicker, and comboboxes

*Use code borrowed from Nemo

*revert x11 icon position handling to the older and well tested code
*use the newer code only in wayland where all the screen can already be used

    search popup

    This gets mis-handled in wayland as a standalone window and
    is treated as such (like getting tiled next to the main caja window).
    Ported over from Nemo