src: drop EditorInfoBar for AdwMessageDialog

`EditorInfoBar` is currently used for requesting a user action in case
unsaved changes were recovered, a document was modified by an external
program, or an error occured while trying to load a document. Depending
on the specific case, the user can then chose to discard the changes,
save the recovered document or retry loading the failed document
through buttons in the infobar.

However, this bar can make the window too wide for small displays (e.g.
mobile phones) depending on the user's locale.

In order to solve this problem, and improve the UI behavior on small
displays, EditorInfoBar is dropped completely and replaced with
instances of AdwMessageDialog: this reverts to a more "traditional"
behavior, but ensures the editor window fits mobile phone screens.