Remove close button from modal dialogs

A user reported that accidentally closing dialogs such as the Objectives editor
with the top-right close button can result in data loss, because the dialog
treats this as "Cancel" and does not prompt to save changes. Now the close
button is removed, so a user must explicitly choose to click one of the main
buttons at the bottom.

I don't know for sure whether the wxSYSTEM_MENU style is needed, but since it
was part of the wxDEFAULT_DIALOG_STYLE and I'm not in a position to test its
removal (it only has an effect on Windows according to the docs), it is left in
for now.

libs/wxutil/dialog/DialogBase.cpp

@@ -18,7 +18,7 @@ void DialogBase::_onDelete(wxCloseEvent& ev)
 DialogBase::DialogBase(const std::string& title, wxWindow* parent)
 : wxDialog(parent ? parent : GlobalMainFrame().getWxTopLevelWindow(),
            wxID_ANY, title, wxDefaultPosition, wxDefaultSize,
-           wxDEFAULT_DIALOG_STYLE | wxRESIZE_BORDER)
+           wxCAPTION | wxSYSTEM_MENU | wxRESIZE_BORDER)
 {
     Connect(wxEVT_CLOSE_WINDOW, wxCloseEventHandler(DialogBase::_onDelete),
             nullptr, this);