Wayland window list: buttons overflow unless show desktop applet is also in the panel

[lukefromdc]

A fix is being investigated for this, but for the moment if the window list is overflowing in wayland with mate-panel from recent git master, the WORKAROUND is to put the show desktop applet anywhere in the panel. I will have a fix for this as soon as I can folks!

Expected behavior:
Window list buttons in a wayland panel never overflow and push other applets off the end of the panel regardless of what other applets are in the panel

Actual behavior:
If the Show Desktop applet is in the panel, the window list works normally. If the Show Desktop applet is NOT in the panel however, the window list wayland backed (used for both applets) somehow cannot get the panel available space and the buttons are never shrunk, thus quickly overflowing the window list

MATE general version:
1.28 with all available 1.29 updates

To Duplicate:
1: Set up a mate-wayland session, using any 1.29 version of mate-panel supporting the show desktop applet. This commit was dated Sep 20
The problem probably came from here, as both applets re-use the same backend code but should spawn two instances.

2: Put the window list in the panel if it is not already there
3: Do NOT put the show desktop applet in the panel-the window list will overflow
4: Put the show desktop applet in the panel, restart the panel if necessary. The window list now will work properly

[lukefromdc]

#1532
fixes this. Leaving it out for review for the moment in case anyone has a better way to do this.

Problem proved to be that since the show desktop applet under wayland creates its own wayland tasklist in the background (not displayed), added buttons get counted twice. This is the cause of the double counted buttons I had to compensate for in the last round of window list code. The purpose of this tasklist invocation is to allow sending a signal to all open windows to minimize or unminimize.

This PR counts every button when the show desktop applet is absent, returns to dividing the count by two when the show desktop applet is present.

If someone knows a better way either for the show desktop applet in wayland to send the necessary signal (without needing a tasklist invocation) or a way to directly suppress the double counting of the buttons (which must be present in the hidden tasklist to receive the signals and control the windows) please speak up. This works but a more elegent fix would be nice.