Dynamically load graphical interface definition

In order to reduce the number of lines in macros with graphical interfaces it is better to define the graphical interface in a ui file that is loaded at runtime. Cf. https://github.com/FreeCAD/FreeCAD-macros/blob/master/FEM/FemAnimateModeShapes.FCMacro as an example.

Things to do:

• Design you graphical interface with Qt Designer
• Add the ui file to the __Files__ variable in the metadata
• In the macro import FreeCADGui as gui
• Later in the macro ui = gui.PySideUic.loadUi('relative_path_to_ui_file') or form, widget = gui.PySideUic.loadUiType('relative_path_to_ui_file')

Running form, widget = gui.PySideUic.loadUiType('relative_path_to_ui_file') with FreeCAD v0.19 on Ubuntu 20.04 requires the manual installation of pyside2uic from PySide2-tools. Commands to be executed in the terminal:

• cd /tmp
• git clone https://github.com/pyside/pyside2-tools
• mkdir build && cd build
• cmake ..
• make
• sudo make install
• sudo mv /usr/local/lib/python3/dist-packages/pyside2uic /usr/local/lib/python3.8/dist-packages