Save and load improvements

You can now save multiple conversions.

Previously, there was only a single save slot that would be overwritten upon running a new conversion.

A conversion will now be saved and given a filename that matches the image used in the conversion. You can press the "load last" button to load the last saved session, or you can select from all previous saved files by pressing the "load" button.

Folders are automatically created for both conversion and pdf saves. They are created in the same folder as the program exe.

Changes have also been made to the interface.

• The small grid segment present when first launching the program is now gone.
• The conversion button now sits over the progress bar.
• Pressing the save button now gives visual confirmation that the conversion has been saved (progress bar changes to say "Conversion Saved")
• Just like when the grid resets upon changing conversion settings, the progress bar now also resets too.
• The loaded image preview no longer resizes when the resize width control is used to modify the conversion settings.

Full Changelog: v0.06...v0.07