Add convenient display functions and window delete commands

[bmag]

The following functions can be convenient:

• purpose-display-at-bottom: display buffer in a new window at the bottom of the frame
• purpose-display-at-top: likewise at top
• purpose-display-at-right: likewise at right
• purpose-display-at-left: likewise at left

The following variables/arguments will give the user a better control over the new window's size:

• purpose-display-window-at-bottom-height
• purpose-display-window-at-top-height
• purpose-display-window-at-right-width
• purpose-display-window-at-left-width

The following commands can be convient for closing those new windows:

• purpose-delete-window-at-bottom: delete the window at the bottom of the frame. if it's not obvious what is the "bottom" window, signal an error.
• purpose-delete-window-at-top: likewise at top
• purpose-delete-window-at-right: likewise at right
• purpose-delete-window-at-left: likewise at left

As a bonus, this should make it easy to implement popup windows and pinned windows.