Replace the disableTextLayer and enhanceTextSelection options/preferences with a single textLayerMode option/preference

Rather than having two different (but connected) options for the textLayer, I think that it makes sense to try and unify this. For example: currently if `disableTextLayer === true`, then the value of `enhanceTextSelection` is simply ignored.

Since PDF.js version `2.0` already won't be backwards compatible in lots of ways, I don't think that we need to worry about migrating existing preferences here.

web/ui_utils.js

@@ -37,6 +37,12 @@ const RendererType = {
   SVG: 'svg',
 };
 
+const TextLayerMode = {
+  DISABLE: 0,
+  ENABLE: 1,
+  ENABLE_ENHANCE: 2,
+};
+
 // Replaces {{arguments}} with their values.
 function formatL10nValue(text, args) {
   if (!args) {
@@ -87,13 +93,6 @@ PDFJS.maxCanvasPixels = (PDFJS.maxCanvasPixels === undefined ?
 PDFJS.disableHistory = (PDFJS.disableHistory === undefined ?
                         false : PDFJS.disableHistory);
 
-/**
- * Disables creation of the text layer that used for text selection and search.
- * @var {boolean}
- */
-PDFJS.disableTextLayer = (PDFJS.disableTextLayer === undefined ?
-                          false : PDFJS.disableTextLayer);
-
 if (typeof PDFJSDev === 'undefined' ||
     !PDFJSDev.test('FIREFOX || MOZCENTRAL')) {
   /**
@@ -669,6 +668,7 @@ export {
   cloneObj,
   PresentationModeState,
   RendererType,
+  TextLayerMode,
   mozL10n,
   NullL10n,
   EventBus,