Text editor plugin, InputDialog enhancements (#4135)

This plugin mostly sets up a "Text editor>" submenu, that allows
browsing files, creating a new file, and managing a history of
previously opened file for easier re-opening.
It restore previous scroll and cursor positions on re-opening.
Additional "Check lua" syntax button is added when editing
a .lua file, and prevent saving if errors.
The text editing is mainly provided by the enhanced InputDialog.

InputDialog: added a few more options, the main one being
'save_callback', which will add a Save and Close buttons
and manage saving/discarding/exiting.
If "fullscreen" and "add_nav_bar", will add a show/hide keyboard
button to it.
Moved the preset buttons setup code in their own InputDialog
methods for clarity of the main init code.
Buttons are now enabled/disabled depending on context for feedback
(eg: Save is disabled as long as text has not been modified).

Added util.checkLuaSyntax(lua_string), might be useful elsewhere.

frontend/ui/elements/reader_menu_order.lua

@@ -72,6 +72,7 @@ local order = {
         "zsync",
         "news_downloader",
         "send2ebook",
+        "text_editor",
         "----------------------------",
         "more_plugins",
     },