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/inputtext.lua

@@ -30,6 +30,7 @@ local InputText = InputContainer:new{
     parent = nil, -- parent dialog that will be set dirty
     edit_callback = nil, -- called with true when text modified, false on init or text re-set
     scroll_callback = nil, -- called with (low, high) when view is scrolled (cf ScrollTextWidget)
+    scroll_by_pan = false, -- allow scrolling by lines with Pan (needs scroll=true)
 
     width = nil,
     height = nil, -- when nil, will be set to original text height (possibly
@@ -322,6 +323,7 @@ function InputText:initTextBox(text, char_added)
             height = self.height,
             dialog = self.parent,
             scroll_callback = self.scroll_callback,
+            scroll_by_pan = self.scroll_by_pan,
         }
     else
         self.text_widget = TextBoxWidget:new{