app.css - fix sidebar list not having the same color as the bg around…

… the searchbar

appdata.xml - description changes

ListView.vala - Implement search for notes by filtering

Even if a model overrides this, it works, so I'm not complaining.
Notes are searched no matter what the search term is like.

Update screenshot

Note.vala - Different Subtitle format

Now, instead of being just the note's creation date, it's:

<time||weekday||date> · <first note line that has a period>

MainWindow.vala / ListView.vala - Remove all warnings

Let's remove all warnings/criticals of the app.

MainWindow.vala - small change

app.css - Remove active styling on note color palette buttons

This was incorrectly set to show active state, which it shouldn't
because these are not a state, but a choice of the user.