Add Grid Button on the Notebook Toolbar

[firezym]

This implementation is so fantastic, which is just what I need for presentation and code/output comparison.

I suggest to bind the commands to grid button/drop down list on the notebook toolbar, so that the users do not need to remember commands or go to command list to click. The user friendly interface could improve usage of this awesome extension. (BTW, the chrome in windows has already taken the Ctrl+Num hotkeys.)

I drafted a dropdown list concept interface FIY :) It just looks like the drop down list for switching cell type between code and markdown. Hope it could be implemented on this awesome extension :P

[firezym]

@parmentelat I am working on this issue now.

Chat with GPT back and forth and tried not so successfully with a mimic implementation like the official toolbar solution (switch cell types). No clue for that.

After that I discard all previous efforts and switch to lumino widgets and have some progress. PR coming soon...

[parmentelat]

I just realized that, as the result of a copy & paste from a former extension, the current code has a provision in plugin.json for a boolean flag configurable in the settings editor
rather than cleaning this up I think it's best if we keep it, and recycle it so users can choose to have their menubar clobbered with the dropdown or not

[firezym]

I am not familiar with the plugin.json and package related stuff because I am just a starter in the frontend development. I rely on gpt-4 to help me a lot in this field. I can not give much insights in this config setting issue.

BTW, #5 the drop down menu is done in this PR. Please check it out :)

[parmentelat]

done in #5 and available in 0.1.4