Allow the layertree to be expanded by default

[svareg]

Hello,

With this small change we would like to define a new attribute gmf-layertree-isexpanded in order to be able to open a layertree expanded by default using the following code:

<gmf-layertree
    gmf-layertree-dimensions="mainCtrl.dimensions"
    gmf-layertree-map="::mainCtrl.map"
    gmf-layertree-openlinksinnewwindow="false"
    gmf-layertree-isexpanded="true">
</gmf-layertree>

[sbrunner]

@fredj Does it looks good to you?

[fredj]

looks good, thanks !