Partially fixes#2068 .
This assumes the headers are in "include" for all plugins - this is
necessary since the INCLUDES argument of add_imhex_plugin may contain
paths to third party libs, whose headers should not be copied.
To fix this I think it is necessary to add a second argument like
"PRIVATE_INCLUDES", which is included, but not installed.
With this it possible to append the ui plugin for example:
```
INCLUDES
$ENV{IMHEX_SDK_PATH}/lib/plugins/ui/
```
and to link against it:
```
LIBRARIES
/usr/local/lib/imhex/plugins/ui.hexpluglib
```
In a follow-up in the CMake Template for plugins imho there should be
fixed, that:
- you can include plugin includes relative to the SDK Path
- you can link plugins relative to the lib path + without the hexpluglib
or hexplug extension
---------
Co-authored-by: Tim Blume <git@3nd.io>
Co-authored-by: Nik <werwolv98@gmail.com>
4e9f944204