core, gtk: implement host resources dir for Flatpak

[alaviss]

Introduces host resources directory as a new concept: A directory containing application resources that can only be accessed from the host operating system. This is significant for sandboxed application runtimes like Flatpak where shells spawned on the host should have access to application resources to enable integrations.

Alongside this, apprt is now allowed to override the resources lookup logic.

[alaviss]

Changes in v2:

• New FlatpakInfo struct that provides methods for querying information about the current Flatpak.
• Only resolve host paths for termio.Exec instead of everything. This fixes issues with themes as Ghostty cannot access the host paths directly and must use /app paths.

[alaviss]

Changes in v3:

• Dropped FlatpakInfo as it is not very useful outside of the host resources directory case.
• GlobalState will now resolve *resources_dir with apprt.runtime implementation if available.
• Added a new parameter to resourcesDir to request for a host-accessible resource directory if available.
• Added host_resources_dir to GlobalState. I'm not entirely sure whether this is the best way, but it's certainly the cleanest to me.
• Removed additional logic in termio.Exec.

[alaviss]

Changes in v4:

• Rebased to main and drop termio, flatpak: implement process watcher with xev #6658 from commit chain.

[pluiedev]

We try to avoid @hasDecl in new apprt-related code, although I'm not exactly sure what's a better alternative here

[mitchellh]

Its better, but I still don't love this approach...

Calling resourcesDir twice once for guest once for host is not great, especially since 99% of users (today, but a significant number) are not running in a sandboxed mode. Its just waste.

A perhaps better approach would be to return a struct instead so we can get all the info in one call. But stepping back its not clear to me if we ever need both, would it be more useful to redocument resourcesDir as always being host accessible?

[alaviss]

But stepping back its not clear to me if we ever need both

For platforms like Flatpak the host-accessible directory is usually non-accessible in the sandbox, but the sandbox path is not accessible outside.

We need sandbox-accessible paths for things like reading bundled themes, and only termio needs the host-accessible one since that's where host processes are launched. So on Flatpak you'd want both to implement all the features.

[alaviss]

Changes in v5:

• Rebased to main
• New ResourcesDir struct that abstracts over host/app paths selection, removing the need to allocate two paths for non-sandboxed cases, as well as reducing the resourcesDir call count to 1.