[thumbnailer appimage] Fix building with libappimage not in system path

Summary:
The current CMake Config file of libappimage does not specify any
imported target nameed "appimage". Instead it provides a shared lib
target "libappimage" and a static lib target "libappimage_static".
Both though are also broken in that they have targets to further
dependencies in their link list, which though are only defined in the
internal build system, but not provided with the installed CMake Config
file.

Additionally the LIBAPPIMAGE_INCLUDE_DIRS is currently not set,
https://github.com/AppImage/libappimage/pull/17 hopefully fixes this
for future versions.

The previous simple "appimage" only worked if libappimage was installed
to an otherwise known prefix, so include dirs and library paths were among
the ones considered, and the linker would find libappimage via -lappimage.

Test Plan:
Building against current version of libappimage installed to custom prefix
works, as well as building against the patched version.

Reviewers: broulik, TheAssassin, azubieta

Subscribers: kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D17088