For Linux users only!
I can't use ALT + left click to select an object in the ObjectTree!
Four different solutions depending on your Linux version:
1. That's a feature of your window manager. If you're on GNOME using Metacity (default WM), you can go to System > Preferences > Windows and change the "Movement Key" (this will just map it to another combination, of course).
2. If you're on KDE go to "System Settings", then in the Personal section "->Desktop->Window Behavior", there will be "Window Actions". Change your setting for "Modifier Key + left Button" from "Move" to "Nothing" or change your modifier Key to Meta instead of Alt.
3. If you are using unity or Gnome 3 you can use the command line: gconftool --set --type string /apps/metacity/general/mouse_button_modifier "<Super>"
4. If you are using Debian 7 (Gnome 3) launch dconf editor go to org -> gnome -> desktop -> wm -> Preferences and edit mouse-button-modifier from "ALT" to "Super".
5. If you are using Ubuntu 15/16, please try this:
sudo apt-get install compizconfig-settings-manager
then, ( e.g. use Start ) to run CompizConfig Settings Manager.
Go to Window Management, click 'MoveWindow'; click under Initiate Window Move for the mouse icon (vs. the keyboard icon) the <Alt> Button1, there, toggle the 'Super' to green, and toggle the 'Alt' to black.