enh(viz): improve how landmarks are interacted with in SightViewer
Description
Right now, to add a landmark, one must either Ctrl+Left click or long tap. Removing a landmark is only supported by selecting it in the landmark list. It is pretty counterintuitive, and a method akin to #1014 (closed) and #1069 (closed) should be ported to landmarks: create two new buttons to add and remove a landmark by clicking on it.
Proposal
Optional section to give some functional or technical hints
Functional specifications
Two new buttons are created. The first is a "Add landmarks" button. When clicked, the application enters a "Add landmarks" mode, where the user must click on the scene to create landmarks at this position. Clicking on the button again disable this mode. The second is a "Remove landmarks" button. When clicked, the application enters a "Remove landmarks" mode, where the user must click on existing landmarks to delete them. Then again, clicking on the button again disable this mode.
Technical specifications
The "Add landmarks" and "Remove landmarks" buttons must be connected respectively to the "toggleAddLandmarks" and "toggleRemoveLandmarks" of the adaptor::Landmarks of the scenes. In order to allow to select the group the new landmarks should be inserted in, metrics::Landmarks should have a new signal "groupSelected" that should be connected to adaptor::Landmarks with a new slot "setCurrentGroup".
Test plan
GUI tests will be added.