More removable device icon types (Bug 4056)

In xfdesktop-settings the removable devices option now has an
expander allowing the user to show network shares, devices, or
unknown/other volumes (such as mtp devices).
Additonal backdrop cycle options

Xfdesktop can now change the backdrop every x seconds, minutes,
or hours. It can also change every hour on the hour, or every day,
or just during start up.
Add a chronological backdrop cycle option

When cycling backdrops sequentially with a period of hourly, setting
this option causes xfdesktop to map files in the selected directory
to hours of the day. Fewer images in the directory than hours in the
day are handled by repeating images to scale them over the time range.
That means there will be at most 24 images displayed showing the first
image from midnight to one, the second from one until two o'clock,
etc. If there were 6 images in the directory then every image would
be displayed for 4 hour time intervals before advancing. If there's
more images in the folder than hours then they won't be shown.
The entire point is that the user can have darker backdrops shown at
night or do other fancy time of day image selections (i.e. one image
in the morning, one in the afternoon, one in the evening, etc).
Tooltip options in xfdesktop-settings

There's now an option to toggle the display of tooltips on the
desktop as well as a spinbox to choose the preview image size.
The tooltip style is still present and if the user doesn't change
the tooltip size then it will fallback to the style setting, and
then finally default to the DEFAULT_TOOLTIP_SIZE.
Allow the font size to go to 0.

Allow the user to set the font size to 0, at which point no icon
label will be drawn.
Include the chronological option in the combobox
Fix some memory leaks
Correctly display settings defaults

When changing workspaces or monitors in the settings app, make sure
we fall back to xfdesktop's defaults when nothing else has been
configured.
Fix memory leaks in the settings app
Merge branch 'eric/settings-changes'
Use the Path key of desktop files if a path is not set (Bug #8741)
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Fix menu popups via the command line
Fix template sub-menu loading code (Bug 10138)

The old code recursively searched the XDG_TEMPLATES_DIR for files
that can be used as templates. This patch changes it so that it
only searches that directory and limits the number of items shown
to the xfconf property max-template-files which is set to 16 to
keep the sub-menu usable. It also only shows files which are not
marked as hidden or backup to eliminate clutter.
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