LibGUI+Apps: Convert Statusbar Labels to Segments

Segments inherit from Button and let us add clickable widgets
to status bars. This patch also adds proportional, fixed and
autosized modes for segments and lets the status bar consume
all non-clickable segments for override text.

Userland/Applications/TextEditor/TextEditorWindow.gml

@@ -106,6 +106,6 @@
 
     @GUI::Statusbar {
         name: "statusbar"
-        label_count: 2
+        segment_count: 3
     }
 }