Pygubu

* 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)

* Fix for issues alejandroautalan/pygubu-designer154, alejandroautalan/pygubu-designer155

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

* Fix name collisions between widget ids, variables and commands. alejandroautalan/pygubu-designer115
* Fix issue with Delete key in macOS. alejandroautalan/pygubu-designer119
* Fix wiki links. alejandroautalan/pygubu-designer118

* Fix Menubutton code generation. refs alejandroautalan/pygubu-designer151
* Fix pyinstaller hook for python 3.8 refs alejandroautalan/pygubu270

* Make more intuitive design screen labels. alejandroautalan/pygubu-designer87
* Fix options of wrap property for ScrolledText. alejandroautalan/pygubu-designer98
* Improve style of generated code. alejandroautalan/pygubu-designer84, alejandroautalan/pygubu-designer85, alejandroautalan/pygubu-designer89, alejandroautalan/pygubu-designer92
* Use black as formatter for generated code.
* Add a new context command menu to select the current item's parent. alejandroautalan/pygubu-designer97, jrezai
* Fix Toplevel preview issues. jrezai
* Fix issue when changing geometry manager. alejandroautalan/pygubu-designer77, alejandroautalan/pygubu-designer80, jrezai
* Examples updated.