Better detect windows that want floating

Windows matching certain criteria are floated by default. Currently,
only windows of type DIALOG, UTILITY, TOOLBAR, or SPLASH were floated.

This patch introduces more robust detection of what windows should be
floated. Besides aforementioned types, also NORMAL windows are floated
if they are either of fixed size, modal, or marked as skipping pager
or taskbar, as are all other (EWMH) window types except DESKTOP and
DOCK.

Closes ticket #1182.
A fix for the desktop window in KDE