Enable shortcuts with buttons invisible (#1580) (#1800)

* Make `ShortcutsWidget` load all shortcuts

`ShortcutsWidget" now loads shortcuts for all capture widgets,
regardless of enabled state in 'Interface settings'.

Also removed unnecessary indirection which made the API confusing:

- Removed `ConfigHandler::setShortcutsDefault` which was only called
  once on a temporary `ConfigHandler` and hence had no effect

- Removed function `ConfigHandler::shortcuts`

- `ShortcutsWidget` now calls `ConfigShortcuts::captureShortcutsDefault`
  directly instead of through `ConfigHandler::shortcuts`

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

* Enable shortcuts for hidden buttons

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

* Fix crash if ButtonHandler has no buttons

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

* obligatory clang-format

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