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.