LibGUI: Allow the InputBox to be of NonemptyText type

It seems like a lot (most?) places where InputBoxes are used check if
the retrieved string isn't empty anyway - make this be reflected in
the user interface, by disabling (graying out) the "OK" button when
nothing is entered, so empty input isn't a viable option at all.

Userland/Libraries/LibGUI/InputBox.h

@@ -14,6 +14,7 @@ namespace GUI {
 
 enum class InputType {
     Text,
+    NonemptyText,
     Password
 };