Send notifications about sync events and possible errors

[d1vanov]

First need to figure out how to work with notifications in Qt.

[d1vanov]

I was hoping to use snorenotify but unfortunately it's not maintained any longer. The ex-maintainer left a recommendation to use KNotifications instead but I'm somewhat reluctant to rely on a piece of KDE. Not because it's bad in any way but because KDE folks like to change things quite often and maintaining compatibility with multiple versions of KNotifications existing on various distro versions could be a PITA.

Perhaps I should learn the high level architecture of snorenotify and implement my own similar yet much more miniscule solution. The supported notification backends I need are

• Free desktop notifications
• macOS notifications center
• Windows pre 10: some custom notification displaying widget
• Windows 10+: Windows toast notifications

Supporting the dual mechanism on Windows can take a significant amount of time and given that Quentier is primarily targeted for Linux, I might go with a more portable mechanism of a custom notification displaying widget. Unless I find some easy enough way to support both.