(uart, debug) first part of fixes after code review

- remove `uart_status` from eu7_application class
- remove copy&pasted `m_scenariopanel` size settings
- remove unnecessary code `debug_panel::render_section_uart()`
- remove #ifdef from `driveruipanels.h` textlines
- set `data_pending` to false when establishing a new connection

driveruipanels.h

@@ -117,13 +117,8 @@ private:
         m_scenariolines,
         m_eventqueuelines,
         m_powergridlines,
- #ifdef WITH_UART
         m_rendererlines,
         m_uartlines;
- #else
-        m_rendererlines;
- #endif
-
 
 	double last_time = std::numeric_limits<double>::quiet_NaN();