prevent saving of partially loaded notebooks

closes #5363
Add dual mode JS tests
s/key_press/trigger_keydown
s/_is_cell_editor_focused/is_cell_editor_focused
s/_is_cell_on/is_cell_on
Make a few more methods public,
in preparation to move them into the base utils.js
Move code into is_cell_editor_focused
Add '_only' to only cell methods
sep. fucntions to move to util.js
Made *+enter tests more complicated.
Add test for cell type modifier keyboard shortcuts
Add dd tests
Fixed typo in dd test
Added up/down and markdown tests
Fixed state type for new markdown cell
Added ctrl+(j&k), a, and b tests
Added get_cell_text
cutcopyandpaste
Added split merge tests
Moved util functions into util.js