Implements the long-requested batch/headless export
(bugtracker #171, GitHub #309): render a project's diagrams to files
without opening the GUI.
qelectrotech --export-pdf <project.qet> <output.pdf> one multi-page PDF
qelectrotech --export-png <project.qet> <output_dir> one PNG per diagram
qelectrotech --export-svg <project.qet> <output_dir> one SVG per diagram
main.cpp detects an export request before SingleApplication is created (so the
arguments are not forwarded to a running instance), spins up a plain
QApplication for rendering, and exits with the export's status code.
Rendering reuses Diagram::render() over
BorderTitleBlock::borderAndTitleBlockRect(), the same geometry the GUI
print/export path uses, so output matches the editor. Image files are named
NN_Title.<ext>.
New files: sources/cli_export.{h,cpp}, registered in
cmake/qet_compilation_vars.cmake.
Shane Ringrose