TextViewer: add support for long-press on text

As done in DictQuickLookup (more event handlers are
needed to cohabitate well with MovableContainer).
By default, selected word or text is copied to clipboard.
Also provide indexes to any long-press callback, as we'll
need them for View HTML.

frontend/ui/widget/textboxwidget.lua

@@ -2012,7 +2012,10 @@ function TextBoxWidget:onHoldReleaseText(callback, ges)
 
         logger.dbg("onHoldReleaseText (duration:", time.format_time(hold_duration), ") :",
                         sel_start_idx, ">", sel_end_idx, "=", selected_text)
-        callback(selected_text, hold_duration)
+        -- We give index in the charlist (unicode chars), and provide a function
+        -- to convert these indices as in the utf8 text, to be used by caller
+        -- only if needed, as it may be expensive.
+        callback(selected_text, hold_duration, sel_start_idx, sel_end_idx, function(idx) return self:getSourceIndex(idx) end)
         return true
     end
 
@@ -2029,7 +2032,7 @@ function TextBoxWidget:onHoldReleaseText(callback, ges)
 
     local selected_text = table.concat(self.charlist, "", sel_start_idx, sel_end_idx)
     logger.dbg("onHoldReleaseText (duration:", time.format_time(hold_duration), ") :", sel_start_idx, ">", sel_end_idx, "=", selected_text)
-    callback(selected_text, hold_duration)
+    callback(selected_text, hold_duration, sel_start_idx, sel_end_idx, function(idx) return self:getSourceIndex(idx) end)
     return true
 end
 
@@ -2085,4 +2088,14 @@ function TextBoxWidget:_findWordEdge(x, y, side)
     return edge_idx
 end
 
+function TextBoxWidget:getSourceIndex(char_idx)
+    if self._xtext then
+        local utf8 = self._xtext:getText(1, char_idx)
+        return #utf8
+    else
+        local utf8 = table.concat(self.charlist, "", 1, char_idx)
+        return #utf8
+    end
+end
+
 return TextBoxWidget