enable server-side decorations for KDE/Plasma with Wayland

[dtorop]

On Plasma, use server-side decoration to give a themed titlebar, a window shadow, and a wide area for window resizing. Leave existing Gnome behavior: client-side decoration with a custom titlebar, which may allow for hiding titlebar when it is maximized, as per #3315.

Fixes #19947 for Wayland/Plasma. Unfortunately, there still needs to be a fix for a wider area for window resizing for Gnome.

[dtorop]

Note that this PR returns the titlebar of the main window to being consistent with other windows in Plasma.

Before PR:

With this PR:

[TurboGit]

Why only for KDE/Plasma?

[dtorop]

Why only for KDE/Plasma?

Good question. As I understand it, Wayland gives window compositors two models:

• Client-side decoration (CSD): The application is responsible for the titlebar, window resize region, etc. This gives the most control, and is the preferred Wayland way to do things. It requires creating a custom GtkHeaderBar. Gnome, prefers (or requires?) CSD.
• Server-side decoration (SSD): the desktop environment takes care of decorations. SSE the classic X11 model, is the easiest to implement (all you have to do is just set gtk_window_set_title()), and provides a unified look across the desktop. KWin defaults to SSD unless the application explicitly creates a GtkHeaderBar, in which case it switches to CSD.

32764b0 effectively switched darktable to using CSD on both Gnome and KDE. But it wasn't a complete CSD implementation. It didn't allow for a >1px window resize zone. It replaced the standard KDE theming from the titlebar with a chunky unstyled titlebar. My understanding is that this wasn't even to fix things under standard Gnome, but for particular Gnome extensions such as Unite.

So this PR at least brings dt under KDE back to its prior SSD state, which gives workable decorations. The ideal fix could be to fully implement CSD for both Gnome and KDE. This means providing a resize drag area, potentially other niceties (shadows), and thinking carefully about what should actually be in the darktable titlebar.

[dtorop]

Reading more about this. The CSD/SSD paradigm seems to be a bit of a rift between Gnome and KDE, with no clear resolution. It would be possible to go all in on CSD (preferred by Gnome, allowed by KDE). The title bar could be customized to contain what is currently the top panel (darktable logo on left, view chooser on right) as well as (potentially) a close button. The title bar would then be hidden when needed by the panels/top and panels/all hide/show shortcuts.

This would be a more GNOME-ish behavior, and would lose the unified look which KDE gives to applications via SSD. Problems would be:

• Still need to use (CSS?) to make a wider drag region for window resizing.
• For non-Wayland setups would still need to keep the extant top panel code (unless they allow CSD).

[dtorop]

Why only for KDE/Plasma?

Added another commit to address this. Instead of checking for KDE/Plasma/KWin, the code now does a more generic check for whether SSD windows are possible. In the case of non-Wayland, the answer is always yes. In the case of Wayland, it checks for xdg-decoration-unstable-v1 protocol, which implies SSD support.

The result is that we should only use CSD for Gnome/Mutter. All other setups will have the benefits of SSD (>1px window resize region, titlebar which matches the desktop). Before this PR, we used CSD for Wayland regardless of whether SSD was available.

Future work (beyond scope of this PR) would be to make CSD work better in Gnome (proper window resize areas) -- ideally so well that we use CSD for KWin as well. But it isn't there yet!

[dtorop]

Saw Windows build timed out. Don't know enough about it to understand why, though.