make width of navigation area configurable (#155)

this allows users to configure navigation width from config.h. it also
allows disabling the navigation function entirely by using a 0 width.

one extra functionality this adds is being able to define an absolute
width (in pixels) instead of just percentage via `NAV_IS_REL`.

Co-authored-by: NRK <nrk@disroot.org>

config.def.h

@@ -173,6 +173,11 @@ static const button_t buttons[] = {
 	{ 0,            5,                g_zoom,               -1 },
 };
 
+/* true means NAV_WIDTH is relative (33%), false means absolute (33 pixels) */
+static const bool NAV_IS_REL = true;
+/* width of navigation area, 0 disables cursor navigation, */
+static const unsigned int NAV_WIDTH = 33;
+
 /* mouse cursor on left, middle and right part of the window */
 static const cursor_t imgcursor[3] = {
 	CURSOR_LEFT, CURSOR_ARROW, CURSOR_RIGHT