Fix shortcut conflict when one is a default (#1989)

* Fix config checking on startup

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

* Handle conflicts when one shortcut is a default

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

* Fix bugs

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

* Refactor slot in ShortcutsWidget for consistency

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

* Update shortcut table on config file change

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

* Fix bounded int bug in config

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

* Revert changes manually

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

* Reimplement ConfigHandler::shortcut and setShortcut

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

* Handle Return/Numpad-Enter equivalently

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

* Re-enable user-configured Imgur upload shortcut

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

* Document copyAndCloseAfterUpload config option

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>