gedit_replace_dialog_new(): GeditWindow parameter

Currently GeditReplaceDialog contains almost only the UI. It doesn't
interact with the GeditDocuments. The purpose is to move some code from
gedit-commands-search.c to GeditReplaceDialog. The latter is a more
logical place in my opinion.
Move code from gedit-commands-search to GeditReplaceDialog

When the search and replace dialog is shown, get the buffer selection,
and if the selection is small enough, place it in the search entry.