display_if only works with single value in Fieldmanager_Checkboxes

I have two separate fields set to `display_if` based on two separate values in a `Fieldmanager_Checkboxes` field. While this does work as a trigger, only one of the conditional fields can appear at the same time.

If two conditional fields depend on the same set of checkboxes, only one conditional field can be displayed at a time — only the most recently selected checkbox triggers the conditional field.


Here is the basic structure of the fields (excess stuff omitted, just showing relevant fields here):

``` php
$placement_fields = [
'type' => new \Fieldmanager_Checkboxes( esc_html__( 'Type', 'pedestal' ), [
'options' => [
'post_type' => 'Post Type',
'post' => 'Single Post',
],
] ),
'post_type' => new \Fieldmanager_Checkboxes( esc_html__( 'Post Type', 'pedestal' ), [
'display_if' => [
'src' => 'type',
'value' => 'post_type',
],
'options' => $post_type_support,
] ),
'post' => new \Fieldmanager_Autocomplete( esc_html__( 'Post', 'pedestal' ), [
'name' => 'post',
'attributes' => [
'placeholder' => esc_html__( 'Search by post title', 'pedestal' ),
'size' => 50,
],
'display_if' => [
'src' => 'type',
'value' => 'post',
],
'datasource' => new \Fieldmanager_Datasource_Post( [
'query_args' => [
'post_type' => Types::get_post_types_by_supported_feature( 'slots' ),
],
] ),
] ),
];
```