Change theme handling

So far when loading a theme it also overwrote the preferences the user
set.

Lengthy discussion can be found at
https://github.com/profanity-im/profanity/issues/1077

Now we use `/theme load themename` to load the [colours] part of a
themem only.

`/theme full-load themename` will load the complete theme including
preferences set in there.

Regards https://github.com/profanity-im/profanity/issues/1077

src/ui/core.c

@@ -364,7 +364,7 @@ void
 ui_handle_login_account_success(ProfAccount *account, gboolean secured)
 {
     if (account->theme) {
-        if (theme_load(account->theme)) {
+        if (theme_load(account->theme, false)) {
             ui_load_colours();
             if (prefs_get_boolean(PREF_ROSTER)) {
                 ui_show_roster();