Navigate to next/previous declaration

[lars-reimann]

Is your feature request related to a problem? Please describe.
Currently navigation is only possible via the tree view and getting to the next/previous declaration of some type (e.g. function) requires to open a few nodes in the tree.

Describe the solution you'd like
Buttons to navigate to the next/previous declaration in the selection view.
Navigating should be only between elements of the same type, e.g. param1 to param2...
After the last element of a type has been reached, jump to the first element of the next "parent" type. When the last/first element is selected, somehow show that in the buttons. Instead of of "next/previous" the buttons should show "go to beginning/go to end".

Buttons should be on the bottom left and right.

[jofaul]

First/Last element button may be added as well

[lars-reimann]

Alternative idea: As part of #504 I've added the feature that all elements for which the filter truly applies are shown in bold. Additionally, their ascendants and descendants are still shown to provide context:

Rather than having a button that switches between elements of the same level (i.e. next parameter etc.), I would prefer that the next button goes to the next declaration with a bold name and that the previous button goes to the previous one.

If a declaration is selected that is not shown in bold, it should still go to the next bold on in the list (likewise for previous):

#505 would probably not be needed anymore then but let's discuss this.