Added dialog with options for desaturation

Not sure if options for RGBA channels are really needed here, but I kept them from invert colors. We could remove them in the future if they are unneeded.

src/UI/Dialogs/ImageEffects/DesaturateDialog.tscn

@@ -0,0 +1,101 @@
+[gd_scene load_steps=2 format=2]
+
+[ext_resource path="res://src/UI/Dialogs/ImageEffects/DesaturateDialog.gd" type="Script" id=1]
+
+[node name="DesaturateDialog" type="ConfirmationDialog"]
+margin_right = 200.0
+margin_bottom = 70.0
+window_title = "Desaturation"
+resizable = true
+script = ExtResource( 1 )
+
+[node name="VBoxContainer" type="VBoxContainer" parent="."]
+margin_left = 8.0
+margin_top = 8.0
+margin_right = 271.0
+margin_bottom = 260.0
+__meta__ = {
+"_edit_use_anchors_": false
+}
+
+[node name="Preview" type="TextureRect" parent="VBoxContainer"]
+margin_right = 263.0
+margin_bottom = 200.0
+rect_min_size = Vector2( 200, 200 )
+expand = true
+stretch_mode = 6
+
+[node name="RGBAContainer" type="HBoxContainer" parent="VBoxContainer"]
+margin_top = 204.0
+margin_right = 263.0
+margin_bottom = 224.0
+alignment = 1
+
+[node name="RButton" type="Button" parent="VBoxContainer/RGBAContainer"]
+margin_left = 83.0
+margin_right = 103.0
+margin_bottom = 20.0
+hint_tooltip = "Modify Red Channel"
+mouse_default_cursor_shape = 2
+toggle_mode = true
+pressed = true
+text = "R"
+
+[node name="GButton" type="Button" parent="VBoxContainer/RGBAContainer"]
+margin_left = 107.0
+margin_right = 129.0
+margin_bottom = 20.0
+hint_tooltip = "Modify Green Channel"
+mouse_default_cursor_shape = 2
+toggle_mode = true
+pressed = true
+text = "G"
+
+[node name="BButton" type="Button" parent="VBoxContainer/RGBAContainer"]
+margin_left = 133.0
+margin_right = 154.0
+margin_bottom = 20.0
+hint_tooltip = "Modify Blue Channel"
+mouse_default_cursor_shape = 2
+toggle_mode = true
+pressed = true
+text = "B"
+
+[node name="AButton" type="Button" parent="VBoxContainer/RGBAContainer"]
+margin_left = 158.0
+margin_right = 179.0
+margin_bottom = 20.0
+hint_tooltip = "Modify Alpha Channel"
+mouse_default_cursor_shape = 2
+toggle_mode = true
+text = "A"
+
+[node name="OptionsContainer" type="HBoxContainer" parent="VBoxContainer"]
+margin_top = 228.0
+margin_right = 263.0
+margin_bottom = 252.0
+
+[node name="SelectionCheckBox" type="CheckBox" parent="VBoxContainer/OptionsContainer"]
+margin_right = 160.0
+margin_bottom = 24.0
+mouse_default_cursor_shape = 2
+pressed = true
+text = "Only affect selection"
+
+[node name="AffectOptionButton" type="OptionButton" parent="VBoxContainer/OptionsContainer"]
+margin_left = 164.0
+margin_right = 263.0
+margin_bottom = 24.0
+mouse_default_cursor_shape = 2
+text = "Current cel"
+items = [ "Current cel", null, false, 0, null, "Current frame", null, false, 1, null, "All frames", null, false, 2, null, "All projects", null, false, 3, null ]
+selected = 0
+[connection signal="about_to_show" from="." to="." method="_on_FlipImageDialog_about_to_show"]
+[connection signal="confirmed" from="." to="." method="_on_FlipImageDialog_confirmed"]
+[connection signal="popup_hide" from="." to="." method="_on_FlipImageDialog_popup_hide"]
+[connection signal="toggled" from="VBoxContainer/RGBAContainer/RButton" to="." method="_on_RButton_toggled"]
+[connection signal="toggled" from="VBoxContainer/RGBAContainer/GButton" to="." method="_on_GButton_toggled"]
+[connection signal="toggled" from="VBoxContainer/RGBAContainer/BButton" to="." method="_on_BButton_toggled"]
+[connection signal="toggled" from="VBoxContainer/RGBAContainer/AButton" to="." method="_on_AButton_toggled"]
+[connection signal="toggled" from="VBoxContainer/OptionsContainer/SelectionCheckBox" to="." method="_on_SelectionCheckBox_toggled"]
+[connection signal="item_selected" from="VBoxContainer/OptionsContainer/AffectOptionButton" to="." method="_on_AffectOptionButton_item_selected"]