~/.config/dwm: Add shortcut to put firefox into fullscreen

Useful for reading articles without any distractions, and for watching
video.

This shouldn't be necessary, but seems to be, because of using dwm *and*
Firefox with several addons.

The `windowfocus` command also doesn't seem to work when Firefox already
has focus, this might be another issue with dwm.  Instead, we pass
`--window` to the `key` command directly.

.config/dwm/config.h

@@ -66,6 +66,8 @@ static char dmenumon[2] = "0"; /* component of dmenucmd, manipulated in spawn()
static const char *dmenucmd[] = { "dmenu_run", "-m", dmenumon, "-fn", dmenufont, "-nb", col_gray1, "-nf", col_gray3, "-sb", col_cyan, "-sf", col_gray4, NULL };
static const char *termcmd[]  = { "/usr/bin/sakura", NULL };
static const char *fullscreenbrowsercmd[] = { "toggle-fullscreen.sh", NULL };
static const char *editorcmd[] = { "emacsclient", "--alternate-editor=", "--create-frame", NULL };
static const char *writecmd[] = { "emacsclient", "--alternate-editor=", "--create-frame", "--eval", "(find-recent-notes)", NULL };
@@ -113,6 +115,9 @@ static Key keys[] = {
	TAGKEYS(                        XK_9,                      8)
	{ MODKEY|ShiftMask,             XK_q,      quit,           {0} },
	{ MODKEY|ShiftMask,             XK_b,      togglebar,      {0} },
	{ MODKEY|ShiftMask,             XK_b,      spawn,          { .v = fullscreenbrowsercmd } },
	{ MODKEY,                       XK_e,      spawn,          { .v = editorcmd } },
	{ MODKEY,                       XK_w,      spawn,          { .v = writecmd } },