Override QuiverA toolbar button actions with our Quiver🐝 buttons

[enjoysmath]

Currently how it works is:

• QuiverA (original) is included into Quiver🐝 using an iframe tag.
• QuiverA is modified so that upon initialization it does: parent.quiverUi = ui; ie. it sets a global variable in Quiver🐝 to the internal reference to the QuiverA UI singleton.
• QuiverA is modified so that it contains methods like "user_save_action()" and "user_undo_action()" in the UI class, which contain calls that are a copy/paste of what the old buttons original actions were.
• So then a button in BSS can be set with onclick="quiverUi.user_undo_action()"

So overriding entails creating a user_xyz_action() in UI class of src/ui.js for each button and finally making the function call upon button click.

All these links/buttons have to be connected to a UI.user_xyz_action().