More missing stock icons.

[cscott]

Split from #7939, which already added a number of missing icons.

The following icons are missing from the sugar theme, and would make "native" GTK applications look much better:

In the 'actions' context:

• 'edit-clear', 'edit-cut', 'edit-select-all', 'edit-find', 'edit-find-replace' -- we've got the other edit- icons, so it looks odd that we've got mismatched icons in the set. 'edit-find' should be similar to 'system-search', which we've already got. 'edit-clear' is our favorite "short paintbrush" icon, which should Die.
• 'window-close' - used in file menu as well as tabs
• 'document-save-as' - should be consistent with 'document-save'

In the 'apps' context:

• 'help-browser' - used for the gtk stock help icon, should be consistent with the icon we use for our help activity.

In the 'places' context:

• 'folder' - used for bookmark groups in firefox.