You can now add a button to a window.
You can now position a button.
You can now change the text on a button.
A button now properly reports its default values.
You can now resize a button.
Extracted the common metatable code and put it into the common.create_metatable() function.
Extracted setters for positioning and sizing into common.add_position() and common.add_size() to make it easy to add common functionality to different classes.
You can now position a window.
Classes now have getters, which allows us to return correct values when properties get changed outside of code (e.g. dragging a window changes its x- and y-properties).
Extracted properties for text/title into common.add_text().
Split common.lua into separate files.
Moved files to subfolders to make maintaining gui.reload() unnecessary and to prevent pollution of package.loaded.