Use available size instead of total size to force 1:1 ratio in panels

We use root.height/root.width as the fallback value for applet sizes
in panels; however this does not take in account the margins, so
applets might end up with height=root.height - margins and width=root.width
Instead, we fallback to the available width/height, which does take
margins in account.

(cherry picked from commit 79aa0fad)