Editor: reuse generic proposal widget

Do not close already visible proposal widget if we get new results for
the same base position. Just replace the model of the widget and update
the prefix.

Change-Id: I298aba6eb8177edc17fea783189a2f987dbf15a2
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/texteditor/codeassist/iassistproposal.cpp

@@ -59,8 +59,9 @@ using namespace TextEditor;
     \sa IAssistProposalWidget, IAssistModel
 */
 
-IAssistProposal::IAssistProposal(int basePosition)
-    : m_basePosition(basePosition)
+IAssistProposal::IAssistProposal(Utils::Id id, int basePosition)
+    : m_id(id)
+    , m_basePosition(basePosition)
 {}
 
 IAssistProposal::~IAssistProposal() = default;