feat: Lock tool style

[Doublonmousse]

Fixes #1104

As of now the lock option is added with a overlay popup that allows unlocking the current pen tool (pen or eraser).
What's left to do is

• expose the option to lock/unlock tools somewhere in the UI.

I think it makes sense to add this in the Button Shortcuts portion of the settings with the added benefit of being able to change tools for pen/eraser there, and the dropdown (temporary/ ...) changed to a locked padlock/unlocked padlock toggle button

[Doublonmousse]

Well, I'm kinda stuck with a BorrowMut error for the (hopefully) very least part of code needed to have a functioning first prototype (with potentially some more bugs/cleanup to do, one of which is getting new icons).

[Doublonmousse]

This finally works. Probably nitpicks to be fixed

• change_pen_eraser_style to be merged into an existing function using an optional argument
• change_style_single_mode : same thing
• change_style_pen_eraser_int : same thing
• set_style_single_mode : same thing

Edit : done

[Doublonmousse]

Seems like I still have borrowmut errors on a fresh compile

[Doublonmousse]

Finally working

[Doublonmousse]

I'm thinking that maybe the logic has to be improved for touch inputs.

Currently, if I use the eraser part (locked to the eraser) on the screen then immediately use touch to change the tool, the lock will appear. So maybe I should just consider that such a press has to be considered as a pen input and switches to the pen mode ?