Hi Pilou for v1.0 it is possible by editing the MainWindowLayout.xml file which is in the \ui subfolder inside of MoI's main installation folder.

The second line of the file that says:
<LayoutPanel dock="right">

can be changed to

<LayoutPanel dock="left">

to make that happen in v1.0


But in v2 it is easier with that checkbox setting under Options / General / Side pane position like you mentioned.

- Michael