Add --filesystem=xdg-cache/thumbnails

[travier]

Grant access to thumbnail cache folder in home directory. This is safe as Gwenview already has access to everything (host) right now as it's not fully portal aware yet.

Fixes: #168

[flathubbot]

Started test build 168756

[flathubbot]

Build 168756 failed

[flathubbot]

Started test build 168761

[flathubbot]

Build 168761 failed

[travier]

Looks like we will need an exception: https://docs.flathub.org/docs/for-app-authors/linter/#finish-args-unnecessary-xdg-cache-subdir-mode-access

{
    "errors": [
        "finish-args-unnecessary-xdg-cache-thumbnails-rw-access"
    ],
    "message": "Please consult the documentation at https://docs.flathub.org/docs/for-app-authors/linter"
}

[Justinzobel]

Created flathub-infra/flatpak-builder-lint#551 for exception.

[Justinzobel]

bot, build

[flathubbot]

Queued test build for org.kde.gwenview.

[flathubbot]

Started test build 170991

[flathubbot]

Build 170991 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/154089/org.kde.gwenview.flatpakref

[flathubbot]

Started test build 171000

[flathubbot]

Build 171000 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/154098/org.kde.gwenview.flatpakref