Add an option to choose the default save folder for captures. Refs #335

docs/window/options_window.rst

@@ -99,6 +99,14 @@ This allows you to choose where on disk temporary capture files are stored betwe
 
 By default the system temporary directory is used, but if this lies on drive with limited space, large captures could become a problem so here you can redirect the storage elsewhere.
   
+---------------
+
+  | ``Default save directory for captures`` Default: ``Empty``
+
+This allows you to choose which directory the save dialog will default to when prompting to save a capture file. By default the value is empty, which means that it follows the system behaviour (e.g. to begin on whichever folder was last browsed to in a file dialog).
+
+The folder must exist, it will not be created when browsed to.
+
 ---------------
 
   | ``Allow global process hooking`` Default: ``Disabled``