appimage: Update / fix build-appimage script

* src/appimage/build-appimage.sh: Use variables for
directories and update them.
(copy KDE plugins): Update names of plugin directories.
(okular): Only package poppler plugin.
(linuxdeploy prep): Just run patchelf on all plugins.
* src/appimage/AppRun: Fix used VERSION file names.

While the diff is a bit hard to read due to using
variables instead of the paths this patch
does little functional changes. One difference
is that instead of calling the generic installer
"GnuPG Foo" we now call it Gpg4win, since
it is build as part of Gpg4win and contains
the Gpg4win software for linux. So Gpg4win4lin.