Before this commit, QWeb crashed when it had to deal with a t-call with
multiple sub nodes on the same line, and one text node:
<t t-call="SomeTemplate">
<span>hey</span> <span>hey</span>
</t>
This was because we need to compile the content to be able to extract
all possible t-set t-value statements. But doing so means that we had a
multiple root templates, which caused a crash in this case.
Solving this issue is simple: the sub template is compiled with the flag
allowMultipleRoots set to true.