source: support only relative paths within the app

Dependencies

• [2] NFI42KGX more correct absolute path detection
• [3] JMUD7T3O get rid of ugly side-effects in tests
• [4] OI4FPFIN support drawings in the source editor
• [5] MKPXANB5 bugfix: mouse clicks on file navigator above log browser side
• [6] KKMFQDR4 editing source code from within the app
• [7] ZJOSQFN6 bugfix: path munging on Windows

Change contents

• edit in source.lua at line 262
  [3.573]→[4.173908:173955](∅→∅),[4.173908]→[4.173908:173955](∅→∅),[4.173955]→[2.506:647](∅→∅)
  − end

  − local filename = Editor_state.filename

  − if is_relative_path(filename) then

  − filename = love.filesystem.getWorkingDirectory()..'/'..filename -- '/' should work even on Windows
• edit in source.lua at line 264
  [4.174206]

  [4.174206]
  + print(Editor_state.filename)
• replacement in source.lua at line 269
  [4.174402]→[4.174402:174425](∅→∅)
  − filename=filename,
  [4.174402]

  [4.174425]
  + filename=Editor_state.filename,