Added additional widgets to the status bar.

This commit adds additional labels to the status bar which display the
current line and column number of the cursor, as well as a widget which
will be used to display notifications.

Additionally, this patch adds borders around the various status bar
fields, to visually separate them from each other.
Connected the line/column fields in the status bar.
Made various coding style improvements.
Added a widget for displaying status bar notifications.
Finished implementing the notification label widget.
Added status bar notification when searches wrap.

When a search operation - whether it's a find or a replace - wraps to
the opposite side of the document (depending on whether or not it's
"reversed"), it now displays a notification of this in the main window's
status bar.