LinGui: Add flatpak support (#231)

LinGui: Add flatpak support (#231)

* LinGui: add --flatpak configure option

For building a linux flatpak bundle

* LinGui: add rules to build flatpak repo and bundles

* LinGui: add flatpak compatible icon

* LinGui: add flatpak compatible desktop file

* LinGui: add flatpak appdata xml

* pkg: disable building LinGui when creating cli flatpak

* pkg: clean flatpak build cache when updating source package

* pkg: fix source package build dependencies

Fixes errors when the git ref being packaged changes and you have not
manually deleted stage and pkg build directories.