Always allow Alt as an orbit modifier in the 3D editor

This makes it easier to navigate in 3D when using a graphics tablet.
This fallback modifier will only be available if no other modifier
is using Alt.

This partially addresses
https://github.com/godotengine/godot-proposals/issues/196.

editor/plugins/spatial_editor_plugin.cpp

@@ -965,7 +965,7 @@ void SpatialEditorViewport::_sinput(const Ref<InputEvent> &p_event) {
 				}
 
 				if (b->is_pressed()) {
-					int mod = _get_key_modifier(b);
+					const int mod = _get_key_modifier(b);
 					if (!orthogonal) {
 						if (mod == _get_key_modifier_setting("editors/3d/freelook/freelook_activation_modifier")) {
 							set_freelook_active(true);
@@ -1656,14 +1656,16 @@ void SpatialEditorViewport::_sinput(const Ref<InputEvent> &p_event) {
 
 			if (nav_scheme == NAVIGATION_GODOT) {
 
-				int mod = _get_key_modifier(m);
+				const int mod = _get_key_modifier(m);
 
-				if (mod == _get_key_modifier_setting("editors/3d/navigation/pan_modifier"))
+				if (mod == _get_key_modifier_setting("editors/3d/navigation/pan_modifier")) {
 					nav_mode = NAVIGATION_PAN;
-				else if (mod == _get_key_modifier_setting("editors/3d/navigation/zoom_modifier"))
+				} else if (mod == _get_key_modifier_setting("editors/3d/navigation/zoom_modifier")) {
 					nav_mode = NAVIGATION_ZOOM;
-				else if (mod == _get_key_modifier_setting("editors/3d/navigation/orbit_modifier"))
+				} else if (mod == KEY_ALT || mod == _get_key_modifier_setting("editors/3d/navigation/orbit_modifier")) {
+					// Always allow Alt as a modifier to better support graphic tablets.
 					nav_mode = NAVIGATION_ORBIT;
+				}
 
 			} else if (nav_scheme == NAVIGATION_MAYA) {
 				if (m->get_alt())
@@ -1672,15 +1674,17 @@ void SpatialEditorViewport::_sinput(const Ref<InputEvent> &p_event) {
 
 		} else if (EditorSettings::get_singleton()->get("editors/3d/navigation/emulate_3_button_mouse")) {
 			// Handle trackpad (no external mouse) use case
-			int mod = _get_key_modifier(m);
+			const int mod = _get_key_modifier(m);
 
 			if (mod) {
-				if (mod == _get_key_modifier_setting("editors/3d/navigation/pan_modifier"))
+				if (mod == _get_key_modifier_setting("editors/3d/navigation/pan_modifier")) {
 					nav_mode = NAVIGATION_PAN;
-				else if (mod == _get_key_modifier_setting("editors/3d/navigation/zoom_modifier"))
+				} else if (mod == _get_key_modifier_setting("editors/3d/navigation/zoom_modifier")) {
 					nav_mode = NAVIGATION_ZOOM;
-				else if (mod == _get_key_modifier_setting("editors/3d/navigation/orbit_modifier"))
+				} else if (mod == KEY_ALT || mod == _get_key_modifier_setting("editors/3d/navigation/orbit_modifier")) {
+					// Always allow Alt as a modifier to better support graphic tablets.
 					nav_mode = NAVIGATION_ORBIT;
+				}
 			}
 		}
 
@@ -1727,14 +1731,16 @@ void SpatialEditorViewport::_sinput(const Ref<InputEvent> &p_event) {
 
 		if (nav_scheme == NAVIGATION_GODOT) {
 
-			int mod = _get_key_modifier(pan_gesture);
+			const int mod = _get_key_modifier(pan_gesture);
 
-			if (mod == _get_key_modifier_setting("editors/3d/navigation/pan_modifier"))
+			if (mod == _get_key_modifier_setting("editors/3d/navigation/pan_modifier")) {
 				nav_mode = NAVIGATION_PAN;
-			else if (mod == _get_key_modifier_setting("editors/3d/navigation/zoom_modifier"))
+			} else if (mod == _get_key_modifier_setting("editors/3d/navigation/zoom_modifier")) {
 				nav_mode = NAVIGATION_ZOOM;
-			else if (mod == _get_key_modifier_setting("editors/3d/navigation/orbit_modifier"))
+			} else if (mod == KEY_ALT || mod == _get_key_modifier_setting("editors/3d/navigation/orbit_modifier")) {
+				// Always allow Alt as a modifier to better support graphic tablets.
 				nav_mode = NAVIGATION_ORBIT;
+			}
 
 		} else if (nav_scheme == NAVIGATION_MAYA) {
 			if (pan_gesture->get_alt())