Implement zooming using Ctrl + Mouse wheel in the GridMap editor

The minimum value of the slider was changed to 0.2 as zooming
works in increments of 0.2. This way, the value can go back to 1
after you've reached the slider's minimum value.

modules/gridmap/grid_map_editor_plugin.h

@@ -214,6 +214,7 @@ class GridMapEditor : public VBoxContainer {
 
 	void _text_changed(const String &p_text);
 	void _sbox_input(const Ref<InputEvent> &p_ie);
+	void _mesh_library_palette_input(const Ref<InputEvent> &p_ie);
 
 	void _icon_size_changed(float p_value);