Allow disabling ValueSlider

Maybe we should make theme changes as well when it's disabled, but it's fine for now.

src/UI/Nodes/ValueSlider.gd

@@ -5,6 +5,7 @@ extends TextureProgress
 
 enum { NORMAL, HELD, SLIDING, TYPING }
 
+export var editable := true
 export var prefix: String
 export var suffix: String
 # Size of additional snapping (applied in addition to Range's step).
@@ -31,6 +32,8 @@ func _ready() -> void:
 
 
 func _gui_input(event: InputEvent) -> void:
+	if not editable:
+		return
 	if state == NORMAL:
 		if event is InputEventMouseButton and event.button_index == BUTTON_LEFT and event.pressed:
 			state = HELD

src/UI/Nodes/ValueSlider.tscn

@@ -15,6 +15,8 @@ size_flags_horizontal = 3
 theme_type_variation = "ValueSlider"
 texture_under = ExtResource( 2 )
 texture_progress = ExtResource( 2 )
+tint_under = Color( 0, 0, 0, 1 )
+tint_progress = Color( 0, 0, 0, 1 )
 nine_patch_stretch = true
 stretch_margin_left = 3
 stretch_margin_top = 3
@@ -28,8 +30,10 @@ anchor_bottom = 1.0
 mouse_filter = 2
 custom_styles/read_only = SubResource( 1 )
 custom_styles/normal = SubResource( 1 )
+text = "0"
 align = 1
 editable = false
+selecting_enabled = false
 
 [connection signal="value_changed" from="." to="." method="_on_value_changed"]
 [connection signal="focus_exited" from="LineEdit" to="." method="_confirm_text"]

src/UI/Timeline/AnimationTimeline.tscn

@@ -780,8 +780,6 @@ rect_min_size = Vector2( 207, 24 )
 size_flags_horizontal = 1
 size_flags_vertical = 0
 value = 100.0
-tint_under = Color( 0, 0, 0, 1 )
-tint_progress = Color( 0, 0, 0, 1 )
 prefix = "Opacity:"
 
 [node name="SpacerControl2" type="Control" parent="ScrollContainer/TimelineContainer/OpacityAndTagContainer"]