Pygubu

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

* Modified ui definition file to allow decluttering of widget ids in designer (alejandroautalan/pygubu-designer117)
* Fix initial value for boolean tkvariables (issue alejandroautalan/pygubu268)
* Improved menu code generation (refs alejandroautalan/pygubu-designer103)

* Fix loading of custom widgets

* Hotfix: Fix error loading tkcalendar DateEntry
* Added pyinstaller hook (thanks to gwelch-contegix)

* New plugin engine and API (alpha state)
* Added support for: AwesomeTkinter, tkintertable, tksheet, ttkwidgets,
tkinterweb, tkcalendar.
* Changed project structure to use src folder.

* Fix: Generate regular treeview properties in the Code Script alejandroautalan/pygubu264 (jrezai)