xdpw: added a screencast chooser

[trigg]

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:

[trigg]

For anyone hoping to test this :
systemctl --user restart xdg-desktop-portal-wlr

the xdpw user service needs to be reloaded after installing (or reboot, if you're insane)

[AKArien]

It is on the layer shell. This is not a hard requirement, but a choice I'm willing to be swayed on

A way to draw the users attention to it will likely be essential

If we’re already on it (which i’m not gonna try to sway you on, sounds good to me), just being there as an overlay should be enough, no ? isn’t that what most desktops just do ?