UI: Add support for capturing per-frame screenshots
* Currently these are simply written to a temp directory and then read back in for use as icons for the frames in the API call tree.
* These are not yet saved with the session data.
UI: Debug sessions are now saved and loaded automatically
* Debug session files are now automatically saved in the user's app data folder (ie: ~/.local/share/VoglEditor64/sessions/)
* When a trace file is loaded the UUID is used to look up a session file, which will also point to a corresponding sessiondata folder
* When a trace is closed, the session file is saved along with a sessiondata folder containing all snapshots and screenshots that were taken during the debug session. This information is then re-loaded when the same trace file is opened again.
* The user may no longer explicitly save or load a session file. However the format of the existing data has not changed, so moving it into the /sessions/ folder shown above (and adding the UUID to the session file and folder name) should allow it to still be loaded.