deskutils/libportal: split into slave ports corresponding to GUI backend

The port has been splitted into 1 master and 3 slave ports for those
who want only a subset of the supported backends and don't want
excessive dependency, namely:

• deskutils/libportal: common non-GUI part (master port)
• deskutils/libportal-gtk3: gtk3 backend
• deskutils/libportal-gtk4: gtk4 backend
• deskutils/libportal-qt5: qt5 backend

While here, correct license and make dependency adjustments of
consumer ports.

PR: 269546
Reviewed by: arrowd
Approved by: arrowd (maintainer)