TextEditor: Merge some "assist" related classes

Simplifies the code base by removing one level of unused
and unneeded abstraction.

- Merge {I,Default}AssistInterface to AssistInterface
- Merge {IAssist,Basic}ProposalItem to AssistProposalItem
- Merge {IGenericProposal,BasicProposalItemList}Model to GenericProposalModel

Change-Id: I54ee7b095427383d67a00fc1d87c3808c21d812d
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/texteditor/codeassist/genericproposal.cpp

@@ -28,12 +28,12 @@
 ****************************************************************************/
 
 #include "genericproposal.h"
-#include "igenericproposalmodel.h"
+#include "genericproposalmodel.h"
 #include "genericproposalwidget.h"
 
-using namespace TextEditor;
+namespace TextEditor {
 
-GenericProposal::GenericProposal(int cursorPos, IGenericProposalModel *model)
+GenericProposal::GenericProposal(int cursorPos, GenericProposalModel *model)
     : m_basePosition(cursorPos)
     , m_model(model)
 {}
@@ -73,3 +73,5 @@ void GenericProposal::moveBasePosition(int length)
 {
     m_basePosition += length;
 }
+
+} // namespace TextEditor