Lots of DAP improvements

Summary:

• The UI tells the backend about current stack and scope
• The scope is retained instead of getting reset all the time. The logic for this currently is extremely simple, we just retain the QComboBox::currentIndex value as the current scope.
• UI is now updated less often resulting in a slightly smoother experience. For e.g., after changing the stackframe, the new stackframe will still be selected.
• Fix thread change being requested when thread combobox populates
• Local variables are now lazily loaded
• Type is shown along with variable

See individual commits for details