core: auto-detect terminal background and apply light theme on first start

Detect the terminal background (via COLORFGBG or an OSC 11 query) before
GUI init and, on the first start, automatically apply the built-in
"light" theme when a light terminal is detected.

The function gui_term_theme_is_light returns an int (1 if light, 0 otherwise,
0 being the safe value when detection is unsure).

tests/unit/tests.cpp

@@ -119,6 +119,7 @@ IMPORT_TEST_GROUP(GuiNick);
 IMPORT_TEST_GROUP(GuiNicklist);
 /* GUI - Curses */
 IMPORT_TEST_GROUP(GuiCursesMouse);
+IMPORT_TEST_GROUP(GuiCursesTerm);
 /* scripts */
 IMPORT_TEST_GROUP(Scripts);