* Android: Replace movement buttons with joystick
Replace movement control buttons (arrows at bottom left screen) with virtual joystick.
Joystick has 8 directions (same as keyboard). Basically, just map it to keyboard input.
Joystick applies only on left 1/3 of screen.
Joystick's position can be fixed by enabling fixed_virtual_joystick setting.
Three new images:
(1) placeholder joystick,
(2) joystick container (background), and
(3) joystick cursor.
Remove unused images: movement control buttons (*_arrow.png).
New data type: touch_gui_joystick_move_id
Joystick's fixed position is spaced one button size from bottom and from left of screen.
Remove unused variable: m_joystick_downlocation
