TextEditor: allow scrolling by lines with Pan (#4145)

Feature of ScrollTextWidget, only used for now by TextEditor.
Pan is like Swipe, but wait a bit at end of gesture to release:
the line on which Pan was started will be moved to where Pan is
released.

May conflict with MovableContainer (so not enabled for DictQuickLookup,
where it could have been nice - but it would work only with text
dictionaries, not with HTML ones, as ScrollHtmlWidget can't really
do that).

frontend/ui/widget/textboxwidget.lua

@@ -684,6 +684,32 @@ function TextBoxWidget:scrollUp()
     end
 end
 
+function TextBoxWidget:scrollLines(nb_lines)
+    -- nb_lines can be negative
+    if nb_lines == 0 then
+        return
+    end
+    self.image_show_alt_text = nil
+    local new_line_num = self.virtual_line_num + nb_lines
+    if new_line_num < 1 then
+        new_line_num = 1
+    end
+    if new_line_num > #self.vertical_string_list - self.lines_per_page + 1 then
+        new_line_num = #self.vertical_string_list - self.lines_per_page + 1
+    end
+    self.virtual_line_num = new_line_num
+    self:free()
+    self:_renderText(self.virtual_line_num, self.virtual_line_num + self.lines_per_page - 1)
+    if self.editable then
+        local x, y = self:_getXYForCharPos() -- luacheck: no unused
+        if y < 0 or y >= self.text_height then
+            -- move cursor to first line of visible area
+            local ln = self.height == nil and 1 or self.virtual_line_num
+            self:moveCursorToCharPos(self.vertical_string_list[ln] and self.vertical_string_list[ln].offset or 1)
+        end
+    end
+end
+
 function TextBoxWidget:scrollToTop()
     self.image_show_alt_text = nil
     if self.virtual_line_num > 1 then