UI: Follow HIG for view item selecting/activating

Makes selecting/activating view items (tree and grid view items) behave
as wanted by the guidelines, consistent with many other editors:
https://developer.blender.org/docs/features/interface/human_interface_guidelines/selection/#select-tweaking

Noteworthy:
- View items activate on press again, not on release
- Pose library still only activates poses on click (releasing mouse
  before moving cursor), so dragging can be used to blend poses
- New: Clicking on empty space in a view deselects all
- Redundant activation in interface handlers code is removed

Pull Request: https://projects.blender.org/blender/blender/pulls/146569

scripts/presets/keyconfig/keymap_data/blender_default.py

@@ -1042,7 +1042,7 @@ def km_user_interface(_params):
         ("ui.view_scroll", {"type": 'WHEELUPMOUSE', "value": 'ANY'}, None),
         ("ui.view_scroll", {"type": 'WHEELDOWNMOUSE', "value": 'ANY'}, None),
         ("ui.view_scroll", {"type": 'TRACKPADPAN', "value": 'ANY'}, None),
-        ("ui.view_item_select", {"type": 'LEFTMOUSE', "value": 'CLICK'}, None),
+        ("ui.view_item_select", {"type": 'LEFTMOUSE', "value": 'PRESS'}, None),
         ("ui.view_item_select", {"type": 'LEFTMOUSE', "value": 'PRESS', "ctrl": True},
          {"properties": [("extend", True)]}),
         ("ui.view_item_select", {"type": 'LEFTMOUSE', "value": 'PRESS', "shift": True},

scripts/presets/keyconfig/keymap_data/industry_compatible_data.py

@@ -420,7 +420,7 @@ def km_user_interface(params):
         ("anim.driver_button_remove", {"type": 'D', "value": 'PRESS', "alt": True}, None),
         ("anim.keyingset_button_add", {"type": 'K', "value": 'PRESS'}, None),
         ("anim.keyingset_button_remove", {"type": 'K', "value": 'PRESS', "alt": True}, None),
-        ("ui.view_item_select", {"type": 'LEFTMOUSE', "value": 'CLICK'}, None),
+        ("ui.view_item_select", {"type": 'LEFTMOUSE', "value": 'PRESS'}, None),
     ])
 
     return keymap