texteditor: simplify basetextmark architecture

ITextMark is not abstract anymore and has an icon and a priority.
This means separate breakpoint and location marker classes that
are only "plain" marks with icons and priorities are not needed.

BaseTextMark directly inherits from ITextMark, instead of owning
an ITextMark derived InternalMark.

Also, there is now ITextMark::paint() to make it a bit more flexible
then icon()[->paint()]

src/plugins/debugger/sourceagent.cpp

@@ -74,20 +74,6 @@ using namespace Core;
 namespace Debugger {
 namespace Internal {
 
-class LocationMarkFoo : public TextEditor::ITextMark
-{
-public:
-    LocationMarkFoo() {}
-
-    QIcon icon() const { return debuggerCore()->locationMarkIcon(); }
-    void updateLineNumber(int /*lineNumber*/) {}
-    void updateBlock(const QTextBlock & /*block*/) {}
-    void removedFromEditor() {}
-    void documentClosing() {}
-    TextEditor::ITextMark::Priority priority() const
-        { return TextEditor::ITextMark::HighPriority; }
-};
-
 class SourceAgentPrivate
 {
 public:
@@ -104,9 +90,11 @@ public:
 
 SourceAgentPrivate::SourceAgentPrivate()
   : editor(0)
-  , locationMark(new LocationMarkFoo)
   , producer("remote")
 {
+    locationMark = new TextEditor::ITextMark;
+    locationMark->setIcon(debuggerCore()->locationMarkIcon());
+    locationMark->setPriority(TextEditor::ITextMark::HighPriority);
 }
 
 SourceAgentPrivate::~SourceAgentPrivate()