feat: use a paned look

[Feichtmeier]

• to reduce the number of visual color background areas, use the paned look that can be found in gtk4 apps
• since the right titlebar then has more contrast against the search field, remove the searchfield fill color override, this also...

Fixes #1612
Fixes #1562

	Before	After
light
dark

[Feichtmeier]

CC @anasereijo

[Feichtmeier]

@anasereijo
If you might be concerned about the whitespace, we could also move certain elements out of the bottom left bar into the appbars at the top

(example mockup)
I think a fully fletched dedicated menu entry only for "about" is very uncommon anyways
what modern gnome apps often do, is place a hamburger menu in the top left bar if it is a paned application
which then leads to a setting dialog


(examples and mockups)

[anasereijo]

@Feichtmeier sorry for the delayed reply. We will be revisiting the App Center next cycle (starting mid May). These are great proposals, and we will be looking at this for sure when we start looking at the store again.

Thanks for your input 🙏

[Feichtmeier]

@anasereijo

Just had another look on this, and I think the settings button is fine in the left panel.

The big improvement (imo) with this change is the visual center. Because now the right pane content and headerbar, including the searchbar, are centered around the same axis

[spydon]

Lgtm, can you open an issue for moving the other things around @Feichtmeier ?
EDIT: I opened #1725

[anasereijo]

@Feichtmeier changes approved, it looks much better! Thanks for doing this, and apologies for taking a long time to come back to you.