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.h

@@ -90,6 +90,8 @@ int tattrset(int);
 void tnew(int, int);
 void tresize(int, int);
 void tsetdirtattr(int);
+void tupdatebgcolor(int, int);
+void tupdatefgcolor(int, int);
 void ttyhangup(void);
 int ttynew(const char *, char *, const char *, char **);
 size_t ttyread(void);