builderparser: Don't exit too early on nested custom tags

builderparser: Don't exit too early on nested custom tags

Currently nested custom tags work only as long as the element names differ
from the root one. If it's same, for example:

<condition type="any">
  <condition type="max-width">600</condition>
  <condition type="max-height">600</condition>
</condition>

then it will fail. Meanwhile the same tags wrapped into <conditions> would
work.

The problem is that custom tag parsing is considered finished as soon as we
encounter a closing tag with the same element name. So instead, track the
nesting level.