Improve desktop notifications

[bebehei]

So, this is my try to improve the notification structure in Syncthing GTK.

This adds now actions to the notifications and counts the events folder-wise to have a meaningful "folder finished" message.

@kozec: I added some TODOs in the code, where I have to invoke some callbacks. Could you help me there? Currently, you can't invoke an action to ignore a specific folder.

Fixes #464

[bebehei]

@kozec Thanks for the patches. I've integrated them and tested everything. It's working pretty well on my system.

Could you please review the code or test it for yourself?

Only quesition is if you want to add it somewhere to your PR or should I add it as function to app.py file. 2nd option is probably better.

Well, I've put the code directly into notification.py. Feel free, to push a refactoring commit on this PR, which puts the logic away from notification.py and puts it into a single function of app.py.

[kozec]

I think that merging and moving that bit of code afterwards will be simpler :)

On that note, are you still making changes?

[bebehei]

I think that merging and moving that bit of code afterwards will be simpler :)

Yeah, that was my plan, too!

On that note, are you still making changes?

No, I'm not making changes anymore. (If so, the commits would appear below my last comment).

[kozec]

Tested and merged, thanks :)

[bebehei]

Wow, cool! Thank you!