Pygubu-designer

* New selected indicator in preview.
* Completed menu support for code generator (issue alejandroautalan/pygubu-designer103)
* Decluttering for widget IDs (issue alejandroautalan/pygubu-designer117)

* Hotfix: Fix template path in preferences window alejandroautalan/pygubu-designer147 (jrezai)

* Changed project structure to use src folder.
* Added internal cursor property editor.
* Adapted code to use pygubu plugin engine
* Merged pull request alejandroautalan/pygubu-designer138 from jrezai/master

* Added more examples
* Fix: 'filedialog.askopenfilename' is blocked by 'preferences.dialog' (larryw3i).
* Translations for pygubu strings in pygubu-designer (larryw3i)

* Add option to center the toplevel preview window. PR alejandroautalan/pygubu-designer124 jrezai
* Fix error loading ui file. issue alejandroautalan/pygubu-designer123
* Added option for i18n support in code generated. issue alejandroautalan/pygubu-designer120
* Added option for select main menu for generated app. issue alejandroautalan/pygubu-designer103 (partially fixed)

* Hot fix: Error when loading old UI files. issue alejandroautalan/pygubu-designer123