fix(editor): the templates looked for in the background from the start, the panels don't wait for them

The include bank read every .inc of the scenery directory on the first use, on the main thread: opening the
switch panel or selecting a switch stopped the editor until all of them were read, which on a large scenery
directory under Windows takes long. The bank now starts looking as the editor starts, in the background,
and the main thread only reads the descriptions once the files are read.

- the switch drives, turntables and signals are read again from the bank when it changes; the panels show
  "Looking for the templates..." meanwhile, only the edits which need the templates wait for them
- the sweep tool takes the templates from the bank instead of looking through the scenery directory again
- a folder linked into one of its own subfolders no longer leads the walk in circles
- the log tells how many templates were found and how long it took

application/editormodeselftestsignal.cpp

@@ -61,6 +61,7 @@ editor_mode::selftest_result editor_mode::selftest_signal(std::string const &com
 				values[pair.substr(0, equals)] = pair.substr(equals + 1);
 		}
 		show_track_tab(track_tab::signals);
+		signal_scan();
 		signal_refresh(true);
 		auto &tool{m_signal};
 		auto const standing_named = [&](std::string const &Name) -> signal_standing const * {