Android: Replace movement buttons with joystick (#7126)

* 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

minetest.conf.example

@@ -86,6 +86,15 @@
 #    type: bool
 # continuous_forward = false
 
+#    The length in pixels it takes for touch screen interaction to start.
+#    type: int
+# touchscreen_threshold = 20
+
+#    (Android) Fixes the position of virtual joystick.
+#    If disabled, virtual joystick will center to first-touch's position.
+#    type: int
+# fixed_virtual_joystick = false
+
 #    Enable Joysticks
 #    type: bool
 # enable_joysticks = false