Add UI/UX support for relative mouse mode

In relative mouse mode, the mouse pointer must be "captured" from the
computer.

Toggle (disable/enable) relative mouse mode using any of the hardcoded
capture keys:
 - left-Alt
 - left-Super
 - right-Super

These capture keys do not conflict with shortcuts, since a shortcut is
always a combination of the MOD key and some other key, while the
capture key triggers an action only if it is pressed and released alone.

The relative mouse mode is also automatically enabled on any click in
the window, and automatically disabled on focus lost (it is possible to
lose focus even without the mouse).

app/src/screen.h

@@ -51,6 +51,11 @@ struct screen {
 
     bool event_failed; // in case SDL_PushEvent() returned an error
 
+    bool mouse_captured; // only relevant in relative mouse mode
+    // To enable/disable mouse capture, a mouse capture key (LALT, LGUI or
+    // RGUI) must be pressed. This variable tracks the pressed capture key.
+    SDL_Keycode mouse_capture_key_pressed;
+
     AVFrame *frame;
 };