Replaced InputText for the script component with a Combo (dropdown)

[nepp95]

• Lists all classes in ScriptEngine::GetEntityClasses
• Effectively removes the need to check for an existing class

Describe the issue (if no issue has been made)

While giving feedback when a class does not exist in the assembly, the input text doesn't what classes are available. Replacing this with a dropdown menu (ImGui::Combo / BeginCombo) removes this handicap, while also removing the check for an existing class, since this always will exist. Nothing has been changed to the "Fields" part of the code, but the alignment and the ScriptClassExists check. The alignment causes an annoying unclear overview of the commit.

PR impact (Make sure to add closing keywords)

List of related issues/PRs this will solve:

Impact	Issue/PR
Issues this solves	None
Other PRs this solves	None

Proposed fix

Described in "Describe the issue"

Additional context

Only tested on Windows x64 with the latest master branch. (f8f8e30)