Two arrays of gaps are stored in pertag, switchable by toggle_gaps

With this commit, each tag has two gap values (indexes at
`gaps_previous`, `gaps_current`) that can be interchanged via
`toggle_gaps`.  At initialisation, `gaps_previous` is set to
`default_gaps` and `gaps_current` is set to all 0.

gaplessgrid.c

@@ -15,7 +15,7 @@ gaplessgrid(Monitor *m) {
 		cols = 2;
 	rows = n/cols;
 
-	const int gap_size = m->pertag->gaps[m->pertag->curtag];
+	const int gap_size = m->pertag->gaps_current[m->pertag->curtag];
 	/* window geometries */
 	cw = cols ? m->ww / cols : m->ww;
 	cn = 0; /* current column number */