This PR adds a small GTK app to :

• Present user with a list of outputs & toplevels
• Allow user to select one
• Return this data to xdpw

Streaming toplevels require two extra protocols:

• ext-image-copy-capture-v1
• ext-foreign-toplevel-list-v1

Testing being done with:

• https://gitlab.gnome.org/-/snippets/19
• https://mozilla.github.io/webrtc-landing/gum_test.html

Packages & versions:

• xdg-desktop-portal-wlr-git 0.8.1.r6.ga413366-1
• sway-git 1.12.r7645.c57daaf-1

Quirks:

• It is on the layer shell. This is not a hard requirement, but a choice I'm willing to be swayed on
• • The app will need to remove itself from the list of presented toplevels
• • A way to draw the users attention to it will likely be essential
• stdout MUST NOT contain anything more than the prescribed reply. More likely we'll need to silence GTK warnings and errors or send them only to std:err. Cancel or failure to choose in any way MUST reply with empty stdout
• The chooser MUST detect when required protocols are missing for toplevel selection and present the user with the outputs instead.
• Currently, hotplug outputs is entirely theoretical. Patches welcome

Other additions to consider:

• Some way of warning or preventing the user from making an invalid selection
• Check required protocols for output screenshare too, and warn user if it is impossible
• For people who like having lots of toplevels, we probably need to consider a scrollbox around the flowbox.
• At some point we'll need to get thumbnails for outputs and toplevels on start & tab switch

TLDR: