Make modx.layout.js theme specific

Each manager template must include layout.js file that extends MODx.Layout and register modx-layout xtype.
Changed translation for user key to User.

[[+username]] is not required in top menu, since the top level of menu is hardcoded in template. This rename fixed name for user's combobox.
Added manager theme combobox and added it into xtype combo. Switched manager_theme to new xtype in system settings.
Switch manager_theme's xtype to theme combo box
Fixed processor's path for media source combo box.
Remove paginator in combo box if it's not needed
Changed content disposition combo box to extend MODx.combo.ComboBox and not directly Ext.form.ComboBox
Changed translation for user key to User

Merge remote-tracking branch 'origin/pr/645' into develop

* origin/pr/645:
  Changed translation for user key to User.
Add manager theme selection combo

Merge remote-tracking branch 'origin/pr/646' into develop

* origin/pr/646:
  Switch manager_theme's xtype to theme combo box
  Added manager theme combobox and added it into xtype combo. Switched manager_theme to new xtype in system settings.
Make modx.layout.js theme specific

Merge remote-tracking branch 'origin/pr/647' into develop

* origin/pr/647:
  Make modx.layout.js theme specific
Fixed processor's path for media source combo box

Merge remote-tracking branch 'origin/pr/648' into develop

* origin/pr/648:
  Fixed processor's path for media source combo box.
Remove combo pagination when not needed

Merge remote-tracking branch 'origin/pr/649' into develop

* origin/pr/649:
  Changed content disposition combo box to extend MODx.combo.ComboBox and not directly Ext.form.ComboBox
  Remove paginator in combo box if it's not needed