src/config/rcxml.c: Add support for <devault /> mousebinds

This loads default mousebinds and provides a way to keep config files
simpler whilst allowing user specific binds.

Note that if no rc.xml is found, or if no <mouse><mousebind> entries
exist, the same default mousebinds will be loaded even if the <devault />
element is not provided.

Example usage (with a slight spelling error):

  <mouse>
    <default />
    <context name="Root">
      <mousebind button="Right" action="Press">
        <action name="ShowMenu" menu="desktop-menu" />
      </mousebind>
    </context>
  </mouse>

Co-Authored-By: @johanmalm

Fixes #416

docs/labwc-config.5.scd

@@ -219,6 +219,13 @@ The rest of this man page describes configuration options.
 	- DoubleClick: Two presses within the doubleClickTime.
 	- Drag: Pressing the button within the context, then moving the cursor
 
+*<mouse><default />*
+	Load default mousebinds. This is an addition to the openbox
+	specification and provides a way to keep config files simpler whilst
+	allowing user specific binds.  Note that if no rc.xml is found, or if no
+	<mouse><mousebind> entries exist, the same default mousebinds will be
+	loaded even if the <default /> element is not provided.
+
 ## LIBINPUT
 
 *<libinput><device category="">*

src/config/rcxml.c

@@ -42,6 +42,7 @@ enum font_place {
 };
 
 static void load_default_key_bindings(void);
+static void load_default_mouse_bindings(void);
 
 static void
 fill_keybind(char *nodename, char *content)
@@ -335,6 +336,11 @@ entry(xmlNode *node, char *nodename, char *content)
 		load_default_key_bindings();
 		return;
 	}
+	if (!strcmp(nodename, "devault.mouse")
+			|| !strcmp(nodename, "default.mouse")) {
+		load_default_mouse_bindings();
+		return;
+	}
 
 	/* handle the rest */
 	if (!content) {