Do not skip windows in the switchpanel
If a window has the skip_window_list flag set wmaker currently not only skips
this window from the window list (F11) but also from the switchpanel (TAB).
This patch makes skip_window_list-windows appear in the switchpanel and be
skipped only in the window list, strictly honoring what the name of the flag
is supposed to mean.
The motivation for this is that I have lots of workspaces and almost all of
them has a xterm, so that I set the skip_window_list for the xterms not to
overcrowd the window list. But I still want to alt+TAB to xterms on the current
workspace.
In addition to that, now the "internal window" flag is not checked before
appearing in the switchpanel.