This allows users to open the clock/calendar popup through DBus, which
means we could configure a keybinding for it.

*Note that in the wayland panel the show desktop applet works by using an invocation of the window list backend to send a signal to every window to mimimize or unminimize
*This however causes every added button to be counted twice when the show desktop applet is also used
*Detect the show desktop applet, and only divide the number of window list buttons by 2 when it is present

The monitor origin needs to be taken into account.

*Set a sane default width of 1/3 the total panel with on startup
*This prevents the panel width from coming up zero or one when GTK gets the allocation with no buttons in it

*Always affirmatively show or hide icons and text when adjusting the buttons for width
*Get the box the tasklist->outer box is packed into for width calculations
*Recompute tasklist width both on adding and removing buttons

*Recompute the maximum button width when three fullsize buttons won't fit
*Otherwise short tasklists overflow

*Do not hide text and retain icons, that's not what we do in x11
*Hide the icons first, only hide labels when ellipsized text can no longer fit
*ensure last buttons don't over-expand as windows are closed
*divide available panel space by number of buttons to set size of empty buttons on crowded panel
*This more evenly fills the window list when a crowded window list forces empty buttons
*and allows far more buttons to show before the window list pushed other applets off the panel

The Appointments and Tasks expanders in the calendar popup were
incorrectly linked to the show-calendar-events and show-tasks gsettings.
This caused collapsing the section to toggle the preference, making the
entire section disappear on next popup.

Fixed by adding a new expand-calendar-events and expand-tasks gsettings
keys to track the UI state separately from whether the section is
visible or not.