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/util.lua

@@ -471,7 +471,7 @@ end
 
 --- Replaces invalid UTF-8 characters with a replacement string.
 --
--- Based on <a href="http://notebook.kulchenko.com/programming/fixing-malformed-utf8-in-lua">http://notebook.kulchenko.com/programming/fixing-malformed-utf8-in-lua</a>
+-- Based on http://notebook.kulchenko.com/programming/fixing-malformed-utf8-in-lua
 ---- @string str the string to be checked for invalid characters
 ---- @string replacement the string to replace invalid characters with
 ---- @treturn string valid UTF-8
@@ -677,4 +677,18 @@ function util.urlDecode(url)
     return url
 end
 
+--- Check lua syntax of string
+--- @string text lua code text
+--- @treturn string with parsing error, nil if syntax ok
+function util.checkLuaSyntax(lua_text)
+    local lua_code_ok, err = loadstring(lua_text)
+    if lua_code_ok then
+        return nil
+    end
+    -- Replace: [string "blah blah..."]:3: '=' expected near '123'
+    -- with: Line 3: '=' expected near '123'
+    err = err:gsub("%[string \".-%\"]:", "Line ")
+    return err
+end
+
 return util