Use actual Event property names in ObjectType

### Description
Many data object properties are prefixed with the object name which makes the graphql queries less than self explanatory.

### Motivation
This makes it unnecessarily complicated for a client that expects a valid SourceChangeCreatedEvent as response.

### Exemplification
For instance the following query were you request a SourceChangeCreatedEvent with a specific value for data.gitIdentifier.commitId but in the repsonse object data.gitIdentifier is renamed to data.sourceChangeCreatedGitIdentifier
```
{
sourceChangeCreated(search: "{ 'data.gitIdentifier.commitId':'ae64999f160ddfec29eaa2da77f6fc82b785ac20' }") {
edges {
node {
data {
sourceChangeCreatedGitIdentifier {
commitId
}
}
}
}
}
}
```
Response:
```
{
"data": {
"sourceChangeCreated": {
"edges": [
{
"node": {
"data": {
"sourceChangeCreatedGitIdentifier": {
"commitId": "ae64999f160ddfec29eaa2da77f6fc82b785ac20"
}
}
}
}
]
}
}
}
```

### Benefits
Simplicity, Consistency

### Possible Drawbacks
There might possibly be queries "in the wild" that have already adjusted to this API.