New class CsValaImportBase is quasi abstract base for C# and Vala import

- Move content of umbrello/codeimport/csharp/csharpimport.{h,cpp} to
  umbrello/codeimport/csvalaimportbase.{h,cpp} and adjust class name to
  CsValaImportBase.
- Move umbrello/codeimport/valaimport.{h,cpp} to new subdirectory vala
  and derive the class from CsValaImportBase.
- Copy content of umbrello/codeimport/vala/valaimport.{h,cpp} to
  umbrello/codeimport/csharp/csharpimport.{h,cpp}, restore class name
  CSharpImport, and restore correct body of function fileExtension().
- In umbrello/codeimport/CMakeLists.txt add file csvalaimportbase.cpp
  and adjust to new subdirectory "vala" with file valaimport.cpp.

CCBUG: 397909