Add registry config for default command

* src/gpgex.cc (getCaptionForId, readDefaultEntry): New.
(gpgex_t::QueryContextMenu): Take default cmd from registry.
(show_about): Update copyright year.

This allows users to configure which command gpgex shows
by default on rightclick through the windows registry.

The value is named GpgExDefault in the GNU\GnuPG key.

The default command available on rightclick
of unencrypted files or folders. The value
must be a string value (REG_SZ) with the Number.
Valid values are:

• 0: Help
• 1: Decrypt & Verify
• 2: Decrypt
• 3: Verify
• 4: Sign & Encrypt
• 5: Encrypt
• 6: Sign
• 7: Import
• 8: Create Checksums
• 9: Verify Checksums
• 11: About

• GnuPG-Bug-Id: T5915