Qbs: Add option to specify custom Python pkg-config name (#4039)

I am Gentoo maintainer of Tiled. In Gentoo, there may be many Python 
versions installed at the same time for compatibility reasons, and each
version has separate name in pkg-config.

For example, for Python 3.12 it would be named python-3.12-embed, so 
running pkg-config for python-embed will fail. Adding this option so
it's possible to specify the correct name from build script.

src/plugins/python/python.qbs

@@ -22,7 +22,7 @@ TiledPlugin {
 
     Probes.PkgConfigProbe {
         id: pkgConfigPython3
-        name: "python3-embed"
+        name: project.pythonPkgConfigName
         minVersion: "3.8"
     }
 

tiled.qbs

@@ -18,6 +18,7 @@ Project {
     property bool sentry: false
     property bool dbus: true
     property string openSslPath: Environment.getEnv("OPENSSL_PATH")
+    property string pythonPkgConfigName: "python3-embed"
 
     references: [
         "dist/archive.qbs",