Find a more elegant way of loading Dash apps into WebDash

[ibdafna]

WebDash currently uses a global window variable window.dashApp to refer to and read the Dash app's Python code. The variable is set as a string inside a .ts file:

https://github.com/ibdafna/webdash/blob/985798820bd65fcc776de9a23e768d59ef279d40/src/dash_app/cross_filtering_app.ts#L2

This warrants a more elaborate discussion around what is the "right" way of specifying which app to load and how to load it. Perhaps it would make sense to read a raw .py file instead and store that in a variable during the build process - I am not sure. Another question worth asking is whether users of WebDash would want to have the option to choose which app to load as a part of the user interface, thus opening the possibility of shipping many apps with WebDash and removing the app choice as an implementation detail.