Save the disappearing markers

[dgossow]

RViz added a new functionality to hide non-active controls on Interactive Markers, which unfortunately makes some markers disappear that shouldn't.

Thus, the following controls should have their always_visible flag set to true:

• The virtual gripper in gripper placement mode
• The nav goal robot

@mateiciocarlie are there more?

[mateiciocarlie]

None that come to mind right now...

[KaijenHsiao]

Object mesh in object placement mode.