Add multiple color schemes and key bindings to change them

This commits adds these color schemes:

- the default (dark) st color scheme
- the default (dark) alacritty color scheme
- One Half (dark & light)
- Solarized (dark & light)
- Gruvbox (dark & light)

Select one with Alt+1..8.
Select the next one with Alt+0.
Select the previous one with Ctrl+Alt+0.

st.c

@@ -2171,6 +2171,28 @@ tstrsequence(uchar c)
 	term.esc |= ESC_STR;
 }
 
+void
+tupdatebgcolor(int oldbg, int newbg)
+{
+	for (int y = 0; y < term.row; y++) {
+		for (int x = 0; x < term.col; x++) {
+			if (term.line[y][x].bg == oldbg)
+				term.line[y][x].bg = newbg;
+		}
+	}
+}
+
+void
+tupdatefgcolor(int oldfg, int newfg)
+{
+	for (int y = 0; y < term.row; y++) {
+		for (int x = 0; x < term.col; x++) {
+			if (term.line[y][x].fg == oldfg)
+				term.line[y][x].fg = newfg;
+		}
+	}
+}
+
 void
 tcontrolcode(uchar ascii)
 {