Add lock view button

[Pullusb]

Add a gizmo button in bottom bar to lock view, similar to GP lock frame addon

Avoid unwanted leave of viewport camera.

Note: extended goal, repurpose pan keys when active like in gp lock frame. But need a better implementation (so probably for a separate issue)

[Pullusb]

method using viewport region_3d.lock_rotation.

Thing to try: add an additional keymap to pan with orbit keys upon activation activated (needs testing)

[Pullusb]

Version 1 done in commit e3040f256375d9d4107d15b7df08b0f3972cae03