Hide/disable bottom toolbar actions in certain contexts

[Fs00]

This PR aims to accomplish what #140 tried to do, with some changes based on the feedback received. Now the button to create a folder is disabled when inside a folder (and not hidden anymore), while in Trash section all buttons are replaced with a single action to delete permanently all notes (which has been moved down from the trash notice toolbar, since IMHO its intent wasn't clear enough in that place).

Here's a screencast showcasing the changes: https://streamable.com/5i3qe

About the argument of the independence of the toolbar from the content of the screen, which was brought up in the previous PR, here's a quote directly from the Material Design guidelines:

To support the intent of different sections of an app, the layout and actions of a bottom app bar can be changed to suit each screen.

I think this a perfect case where bottom bar actions should change, since none of the other three apply to the context of the trash (and therefore are confusing for the user).

[Fs00]

@BijoySingh Is there any chance that my PRs will get merged? Can we consider Scarlet an abandoned project?

[Fs00]

Merged in my personal fork.