Problem:
If there are floating windows when `:mksession` runs, the session cannot
be properly restored.
Solution:
Change `:mksession` to skip floating windows. This matches Vim's
treatment of popup windows.
An alternative approach could have `:mksession` save floating windows
that can be _properly_ restored (rather than skip them entirely, which
is what this PR does). While that would seemingly be a more complete
fix, that could present additional issues since floating windows are
ordinarily created by plugins, and they may no longer be properly under
a plugin's control when restored.
closes#18432
3fe6bf3a1e