src/xwayland.c: keep SSD state on unmap/map cycle

src/xwayland.c: keep SSD state on unmap/map cycle

Move SSD related map() work into the !view->been_mapped branch,
similar to src/xdg.c.

Without this patch following series of events restores the initial
state of the SSD when activating the view:
- spawn xcalc
- observe it shows the SSD
- toggleDecorations to hide the SSD (via keybinding or window menu)
- minimize xcalc (via some panel or A-Space)
- activate xcalc (via some panel or A-Tab)
- observe the SSD is back visible