Enable window select for export

[dubstar-04]

Consider window selection for export.

This could form part of a bigger part of work to make design core scriptable.

Allow users to automate drawing operations by running external .js files against a live Design Core instance.

designCore.doc — document-level operations (open, save, import, export)
designCore.db — drawing database access (iterate/query/modify entities)
designCore.ed — editor interaction (prompt user, write to command line, get current selection)
designCore.cmd — programmatic command dispatch (equivalent to typing at the command line)

Example
// Draw a circle at user-picked point
const pt = await designCore.ed.getPoint('Pick centre:');
const radius = await designCore.ed.getReal('Enter radius:');
await designCore.cmd.sendCommand('CIRCLE', pt, radius);

// Iterate entities
for (const entity of designCore.db.entities()) {
if (entity.type === 'Line') {
entity.setProperty('colour', 'red');
}
}