Allow app filter to be unselected (#349)

**What this PR does / why we need it**:
As same as filtering for piped list.

**Which issue(s) this PR fixes**:

Fixes #

**Does this PR introduce a user-facing change?**:
<!--
If no, just write "NONE" in the release-note block below.
-->
```release-note
NONE
```

This PR was merged by Kapetanios.

pkg/app/api/api/web_api.go

@@ -318,11 +318,11 @@ func (a *WebAPI) ListApplications(ctx context.Context, req *webservice.ListAppli
 		},
 	}
 	if o := req.Options; o != nil {
-		if !o.IncludeDisabled {
+		if o.Enabled != nil {
 			filters = append(filters, datastore.ListFilter{
 				Field:    "Disabled",
 				Operator: "==",
-				Value:    false,
+				Value:    !o.Enabled.GetValue(),
 			})
 		}
 		// TODO: Support multiple filtering of Kinds, SyncStatuses and EnvIds

pkg/app/api/service/webservice/service.proto

@@ -182,7 +182,7 @@ message DisableApplicationResponse {
 
 message ListApplicationsRequest {
     message Options {
-        bool include_disabled = 1;
+        google.protobuf.BoolValue enabled = 1;
         repeated model.ApplicationKind kinds = 2;
         repeated model.ApplicationSyncStatus sync_statuses = 3;
         repeated string env_ids = 4;