Add command pushwindow.

Add command to push the current window to another frame.
The current window moves to the selected frame.
The current frame gets assigned a new window.
The focus stays on the current frame (and so changes to the new window
now displayed in the current frame).
Add pushwindow to the man page.
Add documentation for command pushwindow.
Add command pullwindow.

Refactor pullwindow and pushwindow to share code.
Document some concerns to address before merging into trunk.
Documentation for pullwindow.
Fix behavior when push/pull window leaves an empty frame.

Fix frame color bug.
Fix pullwindow keybinding error.
Remove TODO about transient windows, we already did this right.
Merge branch 'master' into move-window

Master has advance. Prepare to send pull request.
Merge branch 'move-window'