tasklist: Auto-adjust icon size (v2)

This merge request is an updated version of !15 (merged) which was reverted due to flickering of tasklist icons.

Changes in respect to version 1 of this patch (that is: in respect to commit 9c9f09ce):

• Fix duplicate application of CSS opacity to the icon of a window group when all windows in the group are minimized (see !28 (comment 23054))
• Update tasklist icon size when panel is resized. The XfcePanelPluginClass struct doesn't have a separate 'icon_size_changed' method - a change in icon size is being signaled through the 'size_changed' method which is also invoked when the panel size changes.
• Add workarounds to ensure that the icon and the label are laid out properly if the icon has been resized
• Minor code cleanup: return G_SOURCE_REMOVE instead of FALSE

Tested in the following environments:

• FreeBSD
• Gentoo
• Xubuntu

---

Auto-adjust icon sizes independent of whether showing labels.

Before:

After:

Additional changes made by this patch:

• Removal of hard-coded constants when drawing the circled number of windows in a group
• Repaint group button icon when its size changes
• Tasklist's default button size is increased from 25 to 32 pixels in order to avoid icons being cropped at the bottom and at the top
• Fix a memory leak

Closes: #90 (closed)

Obsoletes: !28 (closed)

See also: #354 (closed)