Pygubu-designer

* Use point instead of pixels in designer UI. Fix glitches on high dpi screens.
* Enable image auto scaling for high dpi screens.
* Fix filedialog size in high dpi screens (Linux)
* Fix for issue 284
* Improved container layout editor for grid manager.
* Fix window centerig code in code templates.

* Fix python 3.8, 3.9 compatibility.

* Fix: Treeview rows overlap in high dpi screen. refs 283
* Sticky editor: Draw widget bigger on high dpi screen.
* GridSelector: Draw widget bigger on high dpi screen.

* Add backspace key to trigger delete action. (269 Tweety24655)
* Set menu tearoff to false by default.
* In style property editor: show Toolbutton ttk class for Button, Checkbutton and Radiobutton widgets.
* Add builder data_pool parameter in templates.
* Add min and max options to number editors.
* New json_entry property editor.

* Fix for issues 251 and 253
* Drop support for python 3.6 and 3.7 (was already not working)

* Fix issue with project path with windows junction link. refs 249
* Fix code template error.
* Fix error when "live" updating preview properties
* Added new example for EditableTreeview