Library QoL improvements

[sonnyp]

Since the library has grown so much we should add QoL improvements to make it more pleasant and appealing. We already made some very nice improvements:

• Library illustration #522 @bragefuglseth
• library: Show which languages are available in each entry #678 @Diego-Ivan

The Library is currently (ab-)using AdwPreferencesWindow and related widgets. This was good to build something quickly but is now limiting us in what we can do. So let's stop using Adw preferences widgets.

• library: Remove usage of Adw.PreferencesWindow #949 @BharatAtbrat
• Stop using AdwPreferencesPage @BharatAtbrat (we don't need that anymore)
• Stop using AdwPreferencesGroup @BharatAtbrat (simples Gtk.Label will do the trick)
• Stop using AdwPreferencesRow @BharatAtbrat

#958

Once that's done we can make further improvements

• library: Add search bar inside library view #950 @BharatAtbrat (makes search more prominent)
• Add a Category filter library: Add Language and Category filters #967
• Add a language filter library: Add Language and Category filters #967
• Hide the category headers on search or when the category filter is active

For the category and language filter we can take inspiration from #510