[18.0] project_task_timer: update for version 18

[joe1981al]

I found this was not updated for 18 yet. Found that the views had changed and tree view is no longer there for timesheet_ids, it is a list now, so updated tree to list and it works now.

[joe1981al]

project_task_timer/__manifest__.py was changed to update major version from 17 to 18.

project_task_timer/views/project_task_views.xml line 17 was modified to change below

• from <xpath expr="//field[@name='timesheet_ids']/tree/field[@name='unit_amount']"
• to <xpath expr="//field[@name='timesheet_ids']/list/field[@name='unit_amount']"