Icon-grid: Do not use the workarea size for the *.rc file filename

Followup of #47 (closed)

Currently whenever the workarea is changed, (even if only by a single pixel), a new .config/xfce4/desktop/*.rc file will be written to store the icon-grid.

(The size of the workarea currently is part of the filename of the *.rc file. E.g. icons.screen0-2544x999.rc )

This results in many different *.rc files which mostly will be duplicates (if the grid was not changed in between).

Since that already in the past caused issues (see https://bugzilla.xfce.org/show_bug.cgi?id=11266#c123), a symlink was added as fallback, to always point to the latest used *.rc file. (Which is rather a hack, and which as well seems to have issues ... see #58 (closed))

The suggestion of this issue is, to simplify the filename of the *.rc file to E.g. : icons.screen0.rc. That hopefully should fix #58 (closed). Migration will not be required, since the symlink already point to the latest *.rc file. After another xfdesktop major version, we pobably could drop the symlink fix as well.

If no blockers are found and you would be fine with it, I could provide a MR for this after xfce 4.18 is out.