build: Install include folders of plugin libraries to SDK (#2074)

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>

cmake/modules/ImHexPlugin.cmake

@@ -1,4 +1,5 @@
 macro(add_imhex_plugin)
+    setSDKPaths()
     # Parse arguments
     set(options LIBRARY_PLUGIN)
     set(oneValueArgs NAME IMHEX_VERSION)
@@ -28,6 +29,10 @@ macro(add_imhex_plugin)
         endif()
     endif()
 
+    if (IMHEX_PLUGIN_LIBRARY_PLUGIN)
+        install(DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/include/" DESTINATION "${CMAKE_INSTALL_PREFIX}/${SDK_PATH}/lib/plugins/${IMHEX_PLUGIN_NAME}/")
+    endif()
+
     # Define new project for plugin
     project(${IMHEX_PLUGIN_NAME})
 
@@ -151,4 +156,4 @@ macro (enable_plugin_feature feature)
 
     remove_definitions(-DIMHEX_PLUGIN_${IMHEX_PLUGIN_NAME}_FEATURE_${feature}=0)
     add_definitions(-DIMHEX_PLUGIN_${IMHEX_PLUGIN_NAME}_FEATURE_${feature}=1)
-endmacro()
+endmacro()