This PR adds support for RPM packaging of Cutter plugins.

Example packaged plugin: https://github.com/im-0/fedora-rpm.r2ghidra-dec.

Is rsync really needed? Can't we use cp with a more clearer list of files that are needed?

It is possible to copy only those header files that required to build r2ghida-dec, but such approach has downsides:

1. Other plugins may require more headers.
2. The list of required header files may change between versions of Cutter and plugins.

Ideally, this should be fixed in Cutter itself, by adding usual include directory containing all headers required to build any external project.

It is possible to copy only those header files that required to build r2ghida-dec, but such approach has downsides:

Other plugins may require more headers.

We can install all headers, no need to select only the very small subset required by r2ghidra-dec.

The list of required header files may change between versions of Cutter and plugins.

It is ok if it changes between Cutter release, however I think it is better to be more explicit in what we include in the package. I'd prefer having something like:

cp src/{common,core,dialogs,menus,plugins}/*.h

Ideally, this should be fixed in Cutter itself, by adding usual include directory containing all headers required to build any external project.

In any case, let's still wait a little bit for this. I've just talked with upstream and it seems the new release is going to add support for include files installation. Since I need to update both radare2 and cutter-re, it may be worth just waiting a little bit and get this feature "for free". Ok?

Ok!

Now there is a -devel package, but Cutter still searches for plugins only in user's home directory. Small spec change required to fix this.