Store screenshots to the org.gnome.gnome-screenshot auto-save-directory directory, if defined

[Michael Aaron Murphy]

When the `PrintScrn` key is pressed in GNOME Shell, it takes a screenshot and writes it to `$HOME/Pictures`, or otherwise falling back to `$HOME` if that directory is not found. The `gnome-screenshot` tool does a similar action, but also checks for the existence of the `auto-save-directory` gsetting in `org.gnome.gnome-screenshot`. This will allow GNOME Shell to also use that key, and store screenshots in this location when found.

### Implementation Note

I added a `CowPtr` type to make it possible for the `get_screenshot_directory()` function to return a `gchar*` which is either owned or borrowed, and to free a value when it is owned. `g_get_user_special_dir ()` and `g_get_home_dir ()` return `gchar*` values which are owned by GLib, and must not be freed, but `g_settings_get_string` is owned, and must be freed.

Alternatively, I could `g_strdup()` the values to copy them onto the heap.

### Testing

1. Before applying the patch, take a screenshot with the PrintScrn key.
2. A screenshot should now be stored in `$HOME/Pictures`.
3. Create the new screenshot directory, and set the value for gnome-screenshot:
```sh
mkdir $HOME/Screenshots
gsettings set org.gnome.gnome-screenshot auto-save-directory $HOME/Screenshots
```
4. Take the screenshot again, and notice that it stores in `$HOME/Pictures` instead of `$HOME/Screenshots`
5. Apply the patch, and restart `gnome-shell`, then take the screenshot again
6. The screenshot should be written in `$HOME/Screenshots` instead of `$HOME/Pictures`.
7. Delete the gsettings value, and try again, to see that it returns to storing screenshots in `$HOME/Pictures`
```sh
gsettings reset org.gnome.gnome-screenshot auto-save-directory
```

[Sebastien Bacher (seb128)]

Thank you for your bug report and work, the right place to suggest code changes is upstream though, https://gitlab.gnome.org/GNOME/gnome-shell/issues
Can you merge propose your changes there?

[Ubuntu Foundations Team Bug Bot (crichton)]

The attachment "screenshot_dir.patch" seems to be a patch. If it isn't, please remove the "patch" flag from the attachment, remove the "patch" tag, and if you are a member of the ~ubuntu-reviewers, unsubscribe the team.

[This is an automated message performed by a Launchpad user owned by ~brian-murray, for any issues please contact him.]

[Daniel van Vugt (vanvugt)]

Michael,

Please also propose your change here:

  https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests

[Michael Aaron Murphy (mmstick76)]

Updating this to state that GNOME has yet to make a decision on how to approach this, but whatever decision they make will not consist of sharing an application-specific variable with GDM.

They seem to be hinting that they will create a global variable that applications can use, but they've yet to make a decisions on where or how they want it implemented. Still waiting for feedback on the implementation specifics.