closes: #1374 .
Use SPDX short-form identifiers instead of lengthy copyright
header to document per-file license and copyright.
This commit updates all files under src/ directory where applicable
as well as org.flameshot.Flameshot.metainfo.xml.
The core now has a method requestCapture, which receives a
CaptureRequest object with all the needed information.
This reduces code duplication in future features.
