Build improvements + fix for window size/position (macOS) (#5771)

* Allow using relative path in MacOS for development builds
* [macOS] Fixes RT "cache"/"config" folder not located in the right place
In actual "dev" branch, RT "cache"/"config" folder is located in
"~/Application Support/RawTherapee" folder instead of
"~/Library/Application Support/RawTherapee" folder (as in v5.8 release
and discribed in RawPedia). Moreover, "CACHE_NAME_SUFFIX" cMake option
was not anymore considered.
* [macOS] Remember RT window position correctly, fixes #3209

As described in GTK documentation, some OS Windows Manager do not
consider setting window size and position before it is completely
created (which seems to be the case for macOS). In this commit,
restoring window size/position is now done after its creation.
Morevover, macOS menu bar height is now considered.

rtgui/rtwindow.h

@@ -123,6 +123,7 @@ public:
     {
         return is_fullscreen;
     }
+    void setWindowSize ();
     void set_title_decorated (Glib::ustring fname);
     void closeOpenEditors();
     void setEditorMode (bool tabbedUI);